The Iris chip (the extra EDRAM for GPU/L4) is seen by Intel as a performance improvement for mobile solutions (ultrabooks, MS Surface Pro, maybe Macbooks some day?) and not a desktop solution.
But from a consumer point of view, it increases performance (either for graphics or for the whole system if I have a discrete GPU), and it can also help with GPU numerical processing, because you get "free" transfers from memory (L4) to graphics memory by flipping a bit (or a few bits) in hardware. Don't know why this wasn't expanded further, appart from the cost of the embedded RAM.