Freedom of expression is one, perhaps you want to say things and not be associated with your "regular" account, that would maybe be seen by co-workers or potential employers. Another is you want to make a bot to post some stuff but not be seen as actually typed by you.
Different followings/ers as well. You could always do a new account with a new email to do that, so nothing is new here. It's just an easier way to make a different new account.