Syrphidae are, together with bees the most important pollinators of crops and wild flowers and without Syrphidae there would have been way less plants.
Without Syrphidae agriculture would get a hard time and the declines in pollinators should be reversed sooner than later to assure our food supplies.
Many species are generalist pollinators meaning the feed on a wide variety of plant species. However it seems that one individual Syrphid fly has a preference for one species at a certain time and thus will act as a good pollinator.
The habitat and habits of Syrphidae species is very variable between species and so many plants are likely to be pollinated by Syrphidae. Even Orchids (refs 1, 2 and 3) are known to attract Syrphidae, by a very ingenious manner, they namely imitate being infested by aphids on which females are atracted.