I think the largest use case is a second account for NSFW content.

🫣

Lemmy instances have been somewhat unreliable and Mali is taking back their domains. So having options is nice.

Porn accounts.

When Lemmy.world was having issues it was useful to have a backup on another instance.

What’s listed in this thread plus the fact that not all instances update to the newer versions at the same time. Sometimes a newer Lemmy version can, for some reason, break existing api contracts so the apps might break. Being able to switch accounts on another instance allows the app to continue to function if that instance hasn’t upgraded yet.

In my case, I use it to separate nsfw from sfw, and also, I am on a lemmy instance that is purely Star Trek, so I have an account there.