If you mix up developer, fixer and stop (if you use it) before you start then you have the potential to use them in the wrong order.
If you mix the developer first then the stop whilst the developer is in the tank followed by the fixer whilst the stop is in the tank, you eliminate that possibility of error.
Wow, glad you stopped me before I made that mistake. No wonder I never pour the stop into the tanks first. It would have baffled me no end if I bought another beaker and suddenly started doing that all the time.