This "nullhw" Vulkan layer is to disable all rendering and compute commands in the command parsing hardware. This null hardware Vulkan layer was written a year ago but only merged now.
This Vulkan layer for Intel's ANV driver is similar to Intel's "Blackhole" render extension for OpenGL . The function that this null hardware layer serves is for quickly finding CPU bottlenecks within the driver.
So this Vulkan layer isn't useful to end users / gamers directly but in turn its usage by Intel's open-source driver developers should help uncover new areas for performance optimizations.
The layer is now in Mesa Git .