In my way of working, the 'why' can go into the overall PR and the 'what' into the individual commits. Both are important - the reason for changes and a concise summary if what you've done.
I have already worked on multiple projects that got handed to us as a .git/ folder. Commit messages referencing non-existent issues abound.
I now make my whole team ensure that nothing crucial is left to live in the PR alone.