Before this system the best way to play for free was through private servers, which was what I used back years ago when I was an unexperienced player who hand´t start making their own money yet. It was also a great way to experience older versions of WOW, this was before WoW classic was offered as an option.
i know it´s belongs into a gray area in terms of legality, but you gotta understand for those of us who were unsure if the monthly fee was worth it, this was the best way to try it out. Heck, it might have been a factor in convincing people to start playing in the official servers. Some players played in both. Then there are the custom mod servers, who were either used to test out characters builds, or simply to have fun. Some had unofficial races. Some had a faster xp rate for the casual gamer, etc....

But its has somes disadvantages, the population might not be big which makes the lfg system slow, plenty of bugs depending how dedicated and talented the team running the server was, some servers had a p2w system that made things really unbalanced, and if you have to wait a while if you wan´t to play new expansions, since it takes time before someone manages to create a playable server. But hey, it was how I got introduced to wow.