The short answer is: makes it quick (make && make install) to setup all the mutt/fetchmail/abook/notmuch integrated yadayada plus adds whitelisting and folder filtering, both local and remote via sieve.
there can be one for each different account inside Mail/Accounts.
I think Mutt is really great software, but I cannot live without tab completion from my addressbook and I need it without spending 1h to configure it every time, so that's the gist of it...