Increasing the supply of labor, and accepting lower wages and worse working conditions than natives.

Edit because I hit the post limit, in response to a comment on the idea that they are mostly poaching top talent from other countries: In reality, most are either low pay low skill workers for contracting companies like Infosys (largest employers of H1Bs), or fresh grads that are cheaper and less risky to train (because they, in a sense, own them).

Edit edit for serge2k: If you're so great, then I'm sure they won't mind paying extra for you when the visa party is over.