When I first considered signing up (I think it was on Mastodon), I was told how very easy it would be to move from one instance to another so it wouldn’t matter where I sign up - so I chose the first server that caught my eye. It was also implied that I’d need exactly one login for the entire Fediverse.
In hindsight, I’m okay with this not being true but it was a disappointment. At least moving servers (while taking your history with you) should be a lot easier if we’re trying to avoid big instances becoming bigger.
You know how people often assume someone is trolling, simply because their account is new?
Another reason would be to not have a billion dead accounts on servers.