Gravity does cut off if trub clogs the hose and then you would have to start the siphon again which means running downstairs drawing liquid through the line and running back upstairs. If it happens again you could be running up and down the stairs quite often.
With a pump you have control of the flow at your fingertips, you can slow it down and speed it up. If it gets clogged you can re-prime the pump and keep going without running back and forth.
