Its primary advantage was along the lines of console development: Removing overhead to allow greater software-hardware performance. Mantle, an effort of debatable value, first marketed itself as a replacement for Dx11, proclaiming DirectX to be dead. One of the largest obstacles and enablers faced by PC gaming has been DirectX, an API that enables wide-sweeping compatibility (and better backwards compatibility), but also throttles performance with its tremendous overhead.
Consoles have long touted the phrase “close to the metal” as a means to explain that game developers have fewer software-side obstacles between their application and the hardware.