Knowing how valve is it’s probably because someone working at Valve has a Vision Pro and wanted better support for it.

Probably as a learning exercise so they know what developing and shipping for that platform is like.

Given they're currently building a similar product, makes sense to see what the competition is up to

To use this knowledge in their VR/AR down the line? 

It portably wasn’t that hard to do. The ROI of something like this was probably worth it even if only a small amount of people will actually use it.