Reverse Curls and Hammer Curls target, as already mentioned, the brachioradialis.
Hammer Curls are also more awesome than reverse curls because you can use more weight. Most people who frequent the gym don’t understand this, so when they see you tossing around the 70s they’ll think you’re a freak.
Hammer Curls and Reverse Curls are a crucial part of grip training and are important exercises for complete arm development.
If your forearms are small, your triceps and biceps will never get as big as they could get.
And aside from the obvious benifits a strong grip carries (Deadlifts, Olympic Lifts, Rows, Chins, Fights, Pickle Jars) having a stronger grip also allows you to squeeze the bar harder and get more triceps activation in pressing movements so you can use more weight.
I believe that Hammer Curls are also the only curl ever done at Westside Barbell.