The Adam's is a classic pattern that continues to be one of the most popular and effective flies around. It mimics a variety of midges along with mosquitoes, mayflies and even some terrestrials. It also works as a general attractor and prospecting fly. The parachute style lets the fly ride flush with the water and the white post makes it highly visible to anglers.
For whatever reason the purple variant of the fly catches a lot of fish. There are not many purple insects flying around in the US. But the fish love these things. They don't match the hatch. But they do catch fish.