If you were able to get 2 Prophet Velens on the board, would you be able to lesser heal for 8 or shadowform deal 8 damage? If you got 3 Velens on the board (2 faceless manipulators) would you deal 16 damage with shadowform?! Or even 24 damage with double shadowform.
Not to mention the 2-3 7/7s you'd have on the board.
I know it's not going to happen all the time, but if you had a Velen and 2 faceless manipulators in your deck, you could definitely get it going.