Non-reliance on wands is a big one, since watching the movements of an opponent's wand, or disarming them, are combat fundamentals. Being able to cast spells unnoticed is another one (consider the powerful effect of the mid-interrogation Memory Charm in the Order of the Phoenix). Also, it's presumably better training in terms of building up power and skill to cast spells without a crutch.

Besides, many spells don't really need extra power to work, as they have a binary effect (like the Quietus Charm) or typically target objects that can't resist (such as the Vanishing Charm).