I have a slo drain in my last last raft bed, which flows into the sump. It sets the height of the water in all of them. Like rfeiller mentioned....if my pump(s) stop, the water level in the raft beds stay the same.
In my backyard system....I had a sump 'before' I ever built any raft beds. Folks with just raft beds, often don't have a 'sump'. They just pump from the last bed.