Every turn you have a possibility of losing a life and drawing a card, and later in the game you can punish them really hard for not having cards in hand at all
Playing The Fourth Doctor, as portrayed by Tom Baker, will allow the player to look at the top card of their library whenever they like, as well as active the Would you Like A...? ability a reference to this Doctors love of Jelly Babies which enables the player to, once per turn, play a historic land or cast a historic spell from the top of their library
So, it gives them an outlet to reprint some more expensive random cards
It comes with the downside of turning whatever you destroy into a 3/3 beast creature, but its worth it
In the mid-late game of Commander, you can easily cheat the mana costs of a few powerful werewolves in a single turn
You can also find out more about how to reroll.