They seem to be mean little guys. They eat other insects including flies. But, aside from being part of the balance Nature supports, they also keep pests under control.
Robber Fly-Efferia aestuans male with Striped Deer Fly-Chrysops vittatus. The image is a little brighter if you click it.
The deer fly can spread a stomach virus to animals and occasionally to humans so E. aestuans is doing us a big favor. Besides the virus deer flies have a nasty little bite.