It's used to increase throttle response by squirting a jet of gas into the intake when you quickly open the throttle. Seems that it sticking is a very common problem, mine just started sticking randomly and constantly gave me headaches, hell it even caused me to crash into a ditch at one point. I never really did figure out what was wrong, instead I just swapped the carb.
Mine sticked as well. It got cough up on the carb seal. The rubber seal was in the way of the opening so I just oiled up the seal real well and but a heavier spring in the pump (from a pen) and it works just fine now.