Would you use the book for creating a rpg character for a game that already exists or for creating a new game?

If it's the first, while ofc more options is beter, I don't think the depth part is a big deal (depending on how little depth there is) because the background can also develop over time, with the starting background simply providing something to work with as it develops. I don't know how much variation you are talking though. My experience with D&D is that with their different aspects of background some tend to have ten options, some have eight and the rest (ideal, flaw etc) typically have 6. Talking about most of the tables being D6 rolls leads me to suspect it is probably similar. That adds up to quite a few combinations even though more of the tables have only six than not. It can still get repetitive on the individual aspect though, but then that would only be if you play the same background. Maybe that makes the difference too and that's what I'm not seeing when I'm comparing it to D&D. If you always play an entertainer you will get a lot of repetition on the D6 tables after a while, but those tables change with each background. If what you are talking about has tables of 6 for one characteristic but that table is for that characteristic for all of the characters regardless of the rest of their background, then that's probably not enough.

Apart from online RPGs which either don't go this heavily into background or instead has it prewritten for you, I've only really played D&D, but with D&D I've generally had characters with the background that came from D&D itself but it expanded over time. I also expanded on the backstory myself to fit this small one or two sentences bit into a bigger story, as well as added to my character.

With my first character with our current group with the current DM, I put all my stuff in my stats before rolling background and then got my ideal as "providing children for my tribe". My partner got the exact same one but interpreted it differently. His idea was stealing children for food... but I was trying to get pregnant... but I had absolutely terrible charisma so trying to convince people to do that with me was not the easiest, which made for enough of a challenge at times. However, I was also raised by wolves and had a pet wolf and that wolf died and then I questioned everything again because my biological family dumped me in the woods and then wolves raised me and then goblins killed my wolf family (I had goblins as a preferred enemy and made it that I hate them and that's why I prefer killing them as opposed to just I enjoy fighting them and can fight and track them easier), so I stopped trying to have a baby for a bit and questioned being a parent instead. We also took in a goblin that was being held captive by other goblins and it joined our party. That's not my doing but I took advantage of it for character growth.

A warforged warlock who was an entertainer who was only in it for the money and fame and gained their powers from a pact with a unicorn who doesn't like if I wear the same clothes at multiple times I see him (d&d stuff so far) that I played later (new campaign too as we all died in the other one in the end) developed over time too. One of the other characters had a plant familiar and it died and we smoked it and got high and Warforged can get poisoned but are resistant, so then my character got high and discovered eating, and developed a habit of eating (even though as a Warforged they didn't need to) and would eat things, including things that aren't food. I set it up so "food" took up space inside of my character and eventually it would get full and I would throw it all up (I've also seen other people say they did similar things too but didn't have a limit and instead just made it that it could be storage etc in a Warforged but when they get drunk they will bring up whatever they are storing in there). We found a magic banquet that refreshes... with wine... after I'd already eaten quite a bit including 3 foot worth of a 5 foot pole that was prestigidated to taste like chocolate... and I said I drunk some but we then dealt with our long rest and I wasn't sure if I drunk it or not so after our long rest I said I drunk some too, but I didn't realise until I clarified with the DM for my notes, that I had drunk the whole banquet of wine both before and after the long rest (it replenished) so I drunk 20 people's servings of wine, twice. Yeah... my character being a Warforged hadn't really experienced throwing up before, but with 40 serves of wine on top of what they'd already eaten, that definitely happened. Shortly after, this Warforged died to a room that told us it was a trap but we checked it out anyway. The same campaign is still going now with my first Barbarian.

If the book you wrote about was being used to create characters for a new game and they were meant to be more fleshed out, the depth matters more of course.

Working on my own card/rpg game, the pre-made characters have varying amounts of depth between them but their backstories have more depth than the way D&D, and by the sounds of it the book you mentioned, does, but that's because they are individual characters with their own individual names and everything. I'm setting up a way to make custom characters too and I'm not sure what to do there. I could go for more depth or go for less depth and encourage more of the building of story over time. I'm thinking more depth of options while keeping some aspects more open is best because more options offered for each aspect doesn't really reduce the amount of creative freedom there as it is simply more options.

If a game spells out the backstory for you, like some board games that cross over into RPG concepts tend to do or a video game might do, obviously it either needs a lot more depth considered or it needs to consider the backstory for the group at large (ie the race or class) and not tell the individual's backstory. Individual backstory is probably easier in single player in the case of video games. WOW style games are likely to just focus on the backstory of the races / sides / alliances / groups instead of the individual. That makes me think... adding extra ideals and stuff that suit the individual to something like an online game like WOW seems interesting. You'd have your own side goals as well and that would affect how you interacted with players and npcs. Probably tricky to do well, but it's an interesting concept. Idk if that has been done well already in a multiplayer video game format but that sounds interesting.

Either way, stuff like this seems like a useful resource and I see what you mean about it also being useful for writing. I can see how that would help with writing in general. More depth of options is generally always a plus so that sounds like a weak point in the book, but players can build on story without it. If it is literally one D6 table for a characteristic and that's the only table for that characteristic and it isn't multiple tables of the same characteristic for different backgrounds like D&D has, I agree that D6 isn't enough variation. I disagreed at first (though ofc wouldn't argue against more options or depth - I purely disagreed on the importance of it) but then once I realised the difference is that there's one of each of those tables per background with D&D and there probably isn't in the case of what you're saying, I did agree because six options on one characteristic is not enough if that is spread between all types of characters. That's not enough simply because it risks getting boring which means the basic backstory is more boring, but also that being more boring might mean the backstory is less exciting to build on too and the player might not develop the backstory further throughout the game either.

Wow, this became long. Sorry about the wall of text. I wrote about my D&D characters before I realised the difference in terms of different backgrounds greatly increasing the number of options and I didn't want to delete it after that because I still wanted to share it and it still illustrated the expanding on backstory, or creation of new traits of a character that I was talking about.