Developers' efficiency. But MS understanding of "gain" is a crooked one. They want to prevent developers from releasing anything for the competition, so they use tools to make it harder (lock-in tactic). Or if they can't prevent it, they want to tax it (i.e. to make it more expensive). It is clearly crooked.

And I don't mean just to make Vulkan better. I mean MS starting supporting it instead of DX12. Same AMD can produce Vulkan support for them for Xbox quite easily.