Bean animal question?

[tim the toolman]

So I'm researching the bean animal overflow system and I see that it is definitely the way to go but I have a question.
The main overflow down pipe needs to have a ball valve to adjust the flow rate and ensure that the system runs quietly and I understand this part. But why do the other 2 pipes need to have a valve on them when they seem to operate at 100% all the time. Am I missing something regarding the necessity od the other 2 ball valves?