This build is not very effective early game, but in late game this build will always be a success!

The reason I buy will of the ancients is that I like to run when I am getting low on HP, that gives me the opportunity to run away from ganks, get good heals in big fights and gives good support for my team... it simply gives you extreme survivability when it comes to big fights and chasing. For example you can run over minions when getting chased to heal yourself and then turn around to kill the enemy.