How does Vibrio cholerae benefit from infecting its host?
After V.cholerae gets into the human intestine it starts to multiply its numbers, and then becomes virulent after sufficiently expanding its numbers. This virulence drives the diarrhea which in part causes the bacteria to slough off into the intestinal lumen, and then into the external environment again.

So in short it uses the human intestine to increase cell numbers.

See work by Bonnie Bassler for a really fascinating understanding of the complexity of this infection.