When I first started looking around here, I had no particular reason to pick one instance, so I didn't. I initially registered with three (kbin.social, lemmy.world and lemmy.one). I was sort of planning to try them out and compare them before settling on one, but I ended up just rotating through them as the mood hit me, and I still use all three. And in fact, I'm planning on adding a couple more.
The thing I like about using multiple instances is that I can change my experience quickly and easily.
Mostly I go back and forth between kbin.social and lemmy.world, and they're notably different. In the first place, they use different software, so the interfaces are quite different. The kbin software is a bit more feature rich but also a bit harder to get around in while the lemmy software is a bit simpler in both respects. And the instances are notably different, since .world is federated with virtually everyone while kbin.social has defederated from a number of instances, and most notably all of the botfarms.
So kbin.social has less content of generally higher quality, so it feels more serious and sedate, while lemmy.world has more content but a lot of it is botspam, so it feels more hectic and noisy. And I just go to whichever one appeals to me more at the moment.
And I'm actually looking for a couple more. I'd like to find one that's deliberately reserved and sort of scholarly - high standards and serious discussion - and one that's overtly goofy snd lighthearted.
And I have no doubt that if they don't exist, they will.