Like AMD's Mantle API, it promises reduced CPU overhead and lower-level access to graphics hardware. But DirectX 12 won't be limited to one vendor's hardware.
Mantle will work on this hardware as well, and won't be limited to one vendor's operating system. From AMD's Mantle FAQ
As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality. -- Albert Einstein