Raytracing Prepass?

I have been manually porting a shader to HDRP to add raytracing support. While doing that I am using shader graph's generated shader code to inspect which passes that I should be adding.

There are two subshaders, one is the regular raster pipeline-related while the other is totally for DXR passes.
But at the last Pass of first subshader is called "Raytracing Prepass". Seems like even if I don't add that pass, raytracing features like pathtracing and rt reflections do work.

I would like to know what that pass does in particular. Any insight would be appreciated.

 

@SebLagarde any insights?

 

Great! Thanks for the insight.