They can be useful as funnels into smaller, more intimate servers. During the pandemic, a couple of friends and I would play a fair amount of Among Us, which basically requires a full game of 10 people with at least moderate communication skills to be fun. You'd hop into one of these mega servers, join a voice call to fill in your missing players, and often times, if you weren't a foaming-at-the-mouth lunatic, you'd end up being invited to a smaller server to play future games. Once such a server gained critical mass that they no longer needed to resort to randoms for extra players, these private servers would start only growing via mutual acquaintance.

I never would have found the smaller servers, where at this point no one's playing the game we initially met through but are all friends, if not for bigger servers meeting the need of that entrance to the pipeline.