What does HDL do that makes it "good" or beneficial?

Many Functions. Hdl is a class of lipoprotein particles that have a variety of functions which, collectively, reduce risk of cardiovascular disease (cvd). The most notable function is reverse cholesterol transport (rct), the process by which cholesterol is mobilized from areas such as plaques back to the liver. Hdl also is an important antioxidant, anti-inflammatory and immune modulating particle.
Pick up trash. Hdl's are like the garbage cans of the blood. They go out and collect cholesterol that is not where it should be. They take the cholesterol back to the liver. It's a complex process, but that the short version of what they do.