Different instances have different federations, so it makes sense to have a few accounts scattered around different instances. Sometimes instances also go down or are slow, so it's good to have a backup somewhere else (my main account used to be on programming.dev, but it had server issues all the time so I no longer bother going there).

Oh, and as others have mentioned, porn. I personally also have an alt I use when I get a bit too verbose right after taking my adderall haha