If your Zilean build doesn't include Void Staff, the current metagame will eat you alive.

And the amount of damage you take from stuff is:

100 / (100 + MR)

So with 100 MR, Amumu would be taking 100 / 200 damage, 50%.

If he proceeds to stack 100 more MR, it'll be 100/300 damage, 33%.

Which effectively means that:

a) You need to deal 2x the normal amount of damage to kill him, when he has 100 MR.

b) With Void Staff, this amount is reduced. eg. 100/ (100 + 0.6 x 100) = 100/160, which is 0.625%. Therefore, you only need to do 1.6x of the normal damage. Effectively, you need to do 80% of the amount of damage required without Void Staff, so your damage is effectively increased by 25%. The more MR he stacks, the more effective Void Staff is.