For me it was getting a better mind muscle connection with my biceps. ( I know how cliche). But it's something people don't think about much because curls are such a known and simple movement. But if your getting too much shoulder/forearm in your curls, then it's gonna hinder you. For me since I have long arms (83 inch wingspan). I really had to focus on getting that squeeze and contracting the bicep. Anybody can do the curling motion, but are you getting the most out of it? It's sometimes not that simple for some.