What was the need of Matrix?

Why did machines with A.I made THE MATRIX? I think there was no use, since they already have humans and sucking down their energy. Why they would need it?

Please don't give silly answers, I need to know.

Funny thing, in a scene where they have an attack of sentinel (first time in the presence of Neo) they have to put a EMP field around the craft. So

Neo asks: EMP?

Trinity: Electro Magnetic Pulse.

Now, Tom Andreson is a hard core computer geek in The Matrix and he isn't aware of the word? Bull shitt..!