What is Copper Thieving and why use it?
Unfortunately the other 3 answers to the question are incorrect, but helps keeping a common misunderstanding alive :-) 

Thieving is added to the outer layers in order to help a more balanced chemical process for the plating.

Also notice that there is no need to "balance copper" (or stackups for that matter) in modern PCB fabrication to avoid "warped boards".

I wrote about this on my blog recently. You can find other references on the net.