It's about feeding your muscles. On your club ride, you'll burn a mix of fat, glycogen, and digested carbs. Moderate riding tries to move more of the energy production toward fat, so you'll conserve glycogen. Then you'll have more endurance on multiple hard climbs. Plus moderate riding does a lot of other things that can also be done by higher effort riding, but at a lower cost in training load. When I look at my time in zones for a club ride, I'll see a lot of zone 3, then zone 4 time somewhat less than zone 3, then zone 2 time less than that, and just a few minutes in zones 1 and 5. The theory is that you want a pyramid of zone times, broad at the base and narrow at the top. So during the week, I try to create a more sensible pyramid of time in zones by concentrating on zone 2 with a little zone 3. I fill in zone 1 time with recovery rides and hiking, though my zone 1 time is always less than my zone 2. If I'm recovering well, I'll also do some zone 5 work during the week to fill in for what I didn't get on the club ride, if I need to.

This is from a perspective of doing hard club rides or races every weekend. A long time ago I asked Pete Penseyres what I needed to do to be more competitive on the 3rd mountain pass of a ride. He said, "More base." He was right.