The only reason I see would be if you want your posts to reach people you don't know and who don't follow you; they will end up on the local timeline of the instance you're on.
Conversely if you don't want to spend time with setting up relays/subscriptions (which can get you the same thing) and want to easily discover things on the local timeline there, the local GUI helps with that.
IMO that Mastodon's timelines work this way by default for non-private instances is unfortunate as it conflates physical infrastructure with social community too much.
Other than that it should all be the same.
Theoretically you should also be able to set up "relay accounts" on foreign servers that can mirror your profile and just mirror/reshare your content there. It's not something I've seen already so you'd probably have to script your own bot for that.
