That does not mean that easier workout are unproductive. Each training session build on the next. You start out with an easy session, then progress to a medium, and then harder session. That is what "Periodization" is about.
Training sessions are like warm up set at a meet. Your warm up sets prepare you for you 1RM, one repetition max.
That is exactly what your training sessions are. As an example, let's say your "Periodization" program is four weeks.
Week 1 is your first warm set.
Week 2 is your second warm up set.
Week 3 is your third warm up set.
Week 4 is where you squeeze everthhing you can get out of your workout.
That means the day after Week 4 you going to feel it, you body was taxes and you know it.
Week 5 is where you start over. Week 5 is Week 1 of your new "Periodization" program. It is a light easy week. It is a recovery week.
Just because it is an easy week dosen't mean that is it unproductive. It allows restoration to take place.