Maybe because it creates a feeling of safety? He enables you to trust yourself by setting clear boundaries, so you never have to worry or feel wrong about contacting him or reaching out within his set boundaries. It's probably good feeling not having to overthink things because you know if he has a problem about something, you would know about it.

It could also be a stability/structure thing. I know that's why I personally wish I had clearly defined boundaries. I have never had structure in my life and it would probably feel good to have that one thing that I could always rely on remaining the same.