Let me put it another way:
Imagine you have 6 aether this turn and will have 13 aether next turn. You can play your immortal now and get 4 damage turn 1, 8 damage turn 2, 12 damage turn 3, ....
-or-
wait 1 turn to play phase dragon: 0 damage turn 1, 8 damage turn 2, 16 damage turn 3, ....
By the time you play the dragon, you broke even and every turn after that you are getting 4 extra damage/turn.