An intercooler is meant to cool the air coming from the turbo before it enters the intake manifold. Since the turbo is compressing the air, it ends up increasing in temperature. It also inherits some of the heat from the turbo itself, but almost all of it comes from the almost instantaneous compression of the air.

Cooler, denser air = better air/fuel mixture = better detonation.

[edit]
Oh, yeah, I may be wrong in this, so somebody wise please correct me if I am..

Intercoolers also allow you to run higher boost pressures without needing to worry about detonation.

Again, I may be mistaken in this.

Everybody in our city has been driving a lot better ever since I started driving a M1A1.