Besides any potential aerodynamic improvement, wheels are simple bolt-on components that can provide immediate gain. Not only that but unsprung mass (as well as rotating mass) offers even further benefits than sprung components, as well as have a knock-on effect.
Lighter wheels can mean lighter suspension components, shocks, brakes, and so on. It's a virtuous cycle of lightweighting. Finally, for hybrid and electric cars this means extended battery range as well.
Source: Automotive News