Basically when you first get your phone, it automatically connects to 4G (or LTE, as it is on Sprint) in 4G areas. If you're not in one, it automatically switches to 3G. A toggle would allow someone to force their phone to connect to 3G or 4G depending on where you are. So if I'm in a 4G area but I want to just use 3G (to conserve battery and whatnot), I'd install an app or change the settings so that I can only use 3G and not 4G for however long I want it.
If you're in a 3G only area the toggle wouldn't matter since you wouldn't even be able to connect to 4G. Toggles are useful to switch system settings on the fly.