I first knew fediverse through Mastodon, so my answer has more to do with the whole concept of fediverse than with Lemmy alone.

My main reasons initially were the following:

1. I don't really like crowded networks (it's actually a personal trait of mine, derived from the physical fact that I don't really like crowded places at all).
2. I hate when algorithms try to lead me, ruling over what I write and/or ruling over what I read.
3. I like alternative options and the unknown. Between a latin/roman A and B, I often tend to choose a greek Gamma (trying to always think outside boxes).

And I kinda of liked it. Well, Mastodon has been a cemetery, so most of my fediverse interactions happen through Lemmy.

Just out of curiosity: among several Lemmy instances, I specifically chose The Lemmy Club as an instance for having a Lemmy account for a symbolic reason. Back when I was signing up on Lemmy and trying to find a good instance, the initial "thelem" from "thelemmyclub" got to my attention, because at that time I was delving into Aleister Crowley's Thelema (Liber Al Vel Legis, The book of the Law). So "the lemmy club" kinda of resembled "Thelema club" to me. I'm not a Thelemite, at least not entirely, because I'm more inclined towards a syncretic Luciferianism, but I liked the hidden symbolism that I got to see within the instance's name (also it's actually another personal trait of mine, trying to find patterns everywhere at every time, even though it's just a pattern to myself).