The main reason; to allow for expansion and experimentation and the extra ports are handy if you get in to shoot from below where you have valves at the bottom as well as on top. The cost of the extra parts is negligible and I think having them available in case you want them later is worth it
I occasionally play with 6 valves (sometimes more) but most of the stuff I do is 3 or 4 valves with extra valves attached with different colours ready to be swapped in. More recently, I have been doing a lot with one and two valves though.