Right now the user count Lemmys is comparatively tiny when held up against reddit - but the user count isn’t the thing that makes a social media site, it’s the engagement
So even if you’re used to lurking, try to get a little more active! Post memes, vote on posts, talk in the comments, whatever!
If people come here and see activity, content, and discussions, they’re more likely to stay and contribute their own - if they come and see a ghost town, they’ll just go back to reddit
The main benefit is being in total control of your experience on lemmy
If you’re on someone else’s server, you don’t really have any control of federation, and your instance admins could just decide to defederate some instance that you enjoy seeing content from without your say. Similarly you may want to defederate from an instance, but on a public server you’d have to appeal to the admins to do so for the entire server.
Other than that it’s mainly a “because it’s cool” thing