Well power is a combination of speed and strength; if you work both independently it is easier on your system than doing TT type workouts all the time. You can work strength at low power levels by lifting weights at a moderate speed. You can work speed (foot turnover) at a low power levels doing high cadence drills in very easy gears. Combine the ability to push on the pedals hard with the ability to move them fast… voila power! The reduced metabolic stress from those low power workouts allows more of those workouts to be done without risking overtraining syndrome. You can really only do very high power work once a week, and that workout doesn’t improve strength or speed much.