A wasp is a predatory flying stinging insect of the order Hymenoptera and suborder Apocrita that is neither a bee nor an ant. A narrower and simpler but popular definition of the term wasp is any member of the aculeate family Vespidae. Wasps are critically important in natural biocontrol as almost every pest insect species has at least one wasp species that is a predator upon it. Parasitic wasps are increasingly used in agricultural pest control as they have little impact on crops.

Well I'm certainly not an authority on the issue, but I think they primarily pollinate.  I haven't heard of them killing other pests, but I could be wrong.