It means doing sweet spot work and specific intervals as opposed to repeatedly long zone 2 rides. Get more bang for your buck. You get more physiological gains by riding in sweet spot and you can get a higher workload in less time. It’s about being efficient with our training time. Many athletes have jobs, kids, family and etc and we can have them train with more quality so they can spend less time on the bike.
Of course if you can push the upper end of these targeted zones that is always good. As long as you can finish the workout and upcoming workouts.