Perforce says something in their best practices document that says something like you should branch when you need to have different policies governing a code line. It took me a while to understand what this meant and the deep wisdom of it.

As a developer i may want to control what i check-in and when, what i sync and what and when i build. That would be my personal policy and would require a branch, assuming i don't like the idea of just keeping changes in my work area, and not safely tucked away in perforce.