You technically don’t have to have a master bone. But…people use
it because it makes like a whole lot easier. It allows you to movie
the entire rig without messing up any of the other bones’ local positions
as they are animated. So you could have a walk-cycle animation (but do not
have the master bone keyed with the rest of them), let that loop in the NLA
editor and then you could also key the master bone manually to move
where ever you want and the walk cycle with follow. It’s just a master control.
You do not have to key it at all if you don’t want, but having it gives you the
option to do things you could not do without it.
I hope this description wasn’t too all over the place.