The only reason I use Twitter is to keep in touch with the Irish literary scene really. They're hardly going to be on Mastodon anytime soon. So pretty much everything.
All the time spent building your network and all the people who would not be moving with you. So...the network and the social part of a social network.
I have an account on both, at the moment (three separate Mastodon instances, actually).
It's not necessary, one account lets you reply/boost/communicate with anyone.
But if you find an instance catering to a community/topic you like, you may want to create an account there to fully participate in that instance's local timeline.
Seriously are there any even slightly famous people who are primarily/mainly Mastodon? I am not talking just actors, but I follow people like Julia Galef and Joel Spolsky and don't see a point in a social network with people like them or my friends and family.
I seem to recall there being a google doc, or some such list of semi-famous people on a gnu social or mastodon instance...Can't find that list, but here are a few on the core mastodon instance: