Insurance. Some big instances have closed down, others regularly go into maintenance, you never know when one might decide to enshittify itself. Federation gives us the ability to use a different instance if needed, and it costs nothing to have some accounts ready to use should you be unable to use your main one.
Compartmentation. Wouldn’t want to open the account where I’m subscribed mainly to porn on the bus or at work.
Access. Not all instances federate with the same set of instances. Having accounts on instances with different federation policies lets you access more of the fediverse.
I wish lemmynsfw had better performance …