I don't think you can do anything too useful with it really.
As for the why, its mostly because no one is programming anything to make use of them.
They said we'd be using graphics cards for physics but that really never materialized in any significant way.
I think the closest thing to what you're talking about us GPGPU, but i think its uses are generally pretty obscure and meaningless to most users.
I'll admit, I don't know much about it. But if there were any good uses for secondary graphics chips, I'm sure it would be all over sites like nvnews.