The playmat comes with a cube for use as a turn marker on the playmat. As there is always 48 cards in the large deck, why is a turn marker necessary?
It's mainly just an aid to help tell exactly how many turns are remaining. It's only there if you buy the mat, so it's not necessary to the game.
(Also, there is a law card that adds 4 cards to the large deck, but that doesn't change your point since it adds an extra turn.)