Several well known routines Texas Method, Madcows, All Pros contain "light" or "medium" days. If the goal of these days is to rest and recovery, wouldn't someone be better off doing no exercises?
Do these sessions still build muscle when your exercising at 90% or 80% of your "heavy" days? Or are they just to practice form?
The goal of light days is not recovery. They may sometimes be light enough to allow some limited recovery from the previous workout, or they may not. The goal is to lift and gain the benefits of lifting without triggering the drawbacks associated with working close to one's maximum volume or intensity.
Volume days are taxing because they require a great deal of muscle and soft tissue recovery. One needs substantial food and rest after such a workout. That's the trade-off for the strength and muscle growth that high volume triggers.
High-intensity days are taxing to the central nervous system and come with increased risk of injury. One often needs multiple days off after efforts close to 1RM. That's the trade-off for the perspective and strength gain that high intensity work brings.
Light days are still lifting days, in that they provide a degree of volume (and thus muscle growth), form practice (and thus improved motor patterns), while avoiding the pitfalls of high-volume or high-intensity work. Just because one isn't operating above 80 or 90% of one's volume or intensity maximum doesn't mean that the workout is unproductive.
