they can be used to warm up the quads, get that nice burn at the end of the workout OR pre-exhaust the quads so that when you hit squats/legpress you can focus more on the auxilliary muscle groups. ie if your quads are close to spent when you hit your squats, you'll be forced to use more glutes/etc to get the weight up. extensions are a good tool to help build your legs, but you'll never be able to build massive treetrunk legs doig only extensions