More precise movement of the chain when you shift, and therefore more precise shifting.
If the pulleys didn't have teeth, as the deralieur moved over to shift, the chain would fall off the pulley, or go to the edge of the pulley, in either case, shifting would be compromised.
__________________
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1