Because as so many people have pointed out, parallel processing of the GPU is way way faster then the CPU. All apps could make use of the GPU not just graphics and in fact the GPU should just be renamed to processing engine or something.
And it's not about showing off. It's the future. It's throughput is going to be incredible. The Speedy Ram and insane SSD are perfect for pros.
And of course it a specific machine! It's for creating content! You've not said what it is you need a pro machine for. You said in another post you are off to windows? So I am guessing it's not FCPX?
Nope.
Not all apps will in fact make use of dual GPUs. There are some apps that would actually benefit more from dual CPU. So ... I agree, why wouldn't they offer it?
And ... speedy RAM? You know RAM speed doesn't make much of a difference performance wise, right? It's mostly marketing.