You can really be in one of the 3 major categories regarding customizable items: Love them, hate them, or you don't care about them. For me, it's simple, it's nice having them, but I wouldn't pay for them.
The importance of the items I would say it's a combination between how much visible are they in the gameplay. For example, in League of Legends skins have been a big part of the game, even through it only changes the aspect of the characters, many players go crazy for them, especially if that skin changes some of the animations your character performs during the match. A friend of mine, who had played this game for years, purchases a skin just because the basic attack of his character did a different animation.
Personally, I stopped playing League after I had a period of almost 3 years, when occasionally I would play daily even for a few weeks; per total I spend easily 1500 hours in this game, but I never purchased a skin, a reason being the fact that I was a teenager that didn't get "pocket money", my parents would give me only if it was something important that I need, and games weren't counted in this list. The second reason I think is the fact that I haven't sawed any value in them at the time, of course, I won some lootboxes and used what I got from them, but my money were better spend somewhere else.
But moba games aren't the only genre that has customizable, now I spend some of my free time, if I can call it that (Life tip: don't get 2 jobs when you're in college), in a Rainbow six siege match with my friends, because I never play this game alone, and this one is packed with customizable. And don't get me wrong, I don't mind having some color on my weapon, or some new hat for an operator, but the first gun in the game, only used by 3 operators, has 42 skins, this amount is insane in my perspective. Yeah, it's nice to have some diversity, but this game is already 80 GB on install, but receives 35 GB ish updates at every new season, adding one or 2 operators, without any new map or mode.
Here's more info about the massive number of skins in Rainbow link
After a few seasons, if you don't do a fresh installation of the game to get rid of the garbage, that the installer doesn't bother to remove, the installation folder becomes 120-130 GB. This isn't really that much considering the standards today for internet speed, cost per GB of storage, but you store a lot of items, like seasonal skins that can't even be unlocked anymore, just sitting there without occupying a little more space and adding one more piece of code to be shown in the game, only for the players that unlocked it some years ago.
Maybe you're thinking: "well, storing a skin or 2 isn't that bad, it's not really that much storage", but looking at the big picture, there are 126 weapons, each having approx 25-50 skins, some having minor details, like color change, some having some add-ons to the weapons, which looks cooler, having some trinkets attached or maybe a base redesign, which you don't actually see most of the time, spending a lot just looking through the scope.
And then are the operators skins, which you can actually see just in the menu, or that can be sawed only by others during the match.
If in moba games, there's something like a tradition to show off your skin at the beginning of the game before the match actually starts off, moking your enemy that your skin looks better, in shooters like Rainbow, you can barely see your skin, looking through a camera (around the building or deathcam, your choice), others shot you at the first sight of movement, nobody really takes a break to admire how you customized your character.
Considering this is a tactical shooter, operators skins might give you even an advantage, this is where I have to give the credits where are deserved, as the game design of this game only allows you to customize your operator only in the main menu. This might be a reason to just save some memory and make the match load faster for all users, but if you think about it, it also eliminates a certain advantage that users could abuse, this being a way to chose your operator skin just before entering a map, so you can choose something that can camouflage better with the environment. As this isn't the game with the best community, with a lot of tryhards that will camp attackers at spawn, taking away one more advantage from them is welcomed, at least for me. This could also work against you, if you equipped a yellow headset just before playing on a darker map, you just put a more obvious target for the other team.
But enough of Rainbow and shooters, let's go to other genre, a little more skin friendly, what I mean is that you can actually see your skin. This genre would be 2D Fighters, to be more exact I'm talking about Mortal Kombat X.
In this case I just love how they designed the customization system, for more reasons, one being the fact that they added some skins, but there up to 7 per character.
Another reason is that you can unlock everything; and the system for this is actually fun, even through the tile based 3D environment for the crypt isn't the best experience, you still have to find your way through the level to get every item, with In game currency, so I can buy a DLC instead of a skin.
But probably the fact that I prefer Mortal Kombat skins instead of the ones from any other game is the fact that they are made with a lot of passion for the game itself, even through you don't actually play Mortal Kombat for the story, you can see that each skin of a character is story related, a certain point in that character history make him look the way that you see it now. Above that, the interaction with other characters also relate to the skin.
For example if Scorpio appears in revenant skin vs Takeda (Scorpio trainee), Takeda will ask why this appearance, on which Scorpio will respond that he serve Shang Tsung; Takeda will declare Scorpio an enemy to the clan of Shirai Ryu, of which master is Scorpio.
I don't know about you, but this kind of story telling is impressive for me, giving you more information about the relationship between 2 characters in a 5 seconds cutscene before a match.
Customizable items come in different form from game to game, and if in some they are really just for esthetic purpose, in other games can really come in the aid of designers to tell a story. In the end if that piece of content makes you happy, and you think it's worth it, you should buy it, it's really up to you how to spend your money. But if you're interested in my opinion, I would rather buy an indie game that looks interesting than a piece of texture for a character, especially if that "skin" is a change of color.
I've chosen to talk about these particular games because I don't want to talk about something that I didn't play yet, cause then my opinion would be just impressions from I get from others and not something personal.
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