As well as helping a cold start, a choke can clear a blocked carburettor jet on the water: with the revs up a little, pulling the choke out makes a strong vacuum that can draw out a drop of water or dirt. Chokes don't close the carburettor off completely, and many newer motors have prime start enrichment instead, which adds fuel for starting but doesn't work as a choke. Straight after, I show it in action with a rag held over the carburettor.