Its pretty simple.

Ever have a PC get stuck in Sleep Mode? Stuck to where you have to hit the Momentary Switch on your tower (reboot)?

The RasPI is in constant on state when power is applied. Many of the setups will put your RasPI Machine in "HALT STATE" sort of like when your PC goes into "SLEEP STATE". Many times the only way to get The RasPI out of "HALT STATE" is to unplug the plug back in (hard reboot) the system. A momentary ON-OFF Button saves unplugging and plugging back in. Especially if your plugs are in hard to reach spots. Hope this helps.