The lighter flywheel/clutch assemblies are desireable when you want the 'noload' RPM's to come up quickly and for the engine to drop RPM's quickly especially when decellerating. Both conditions you see when running the 'twisties'.

Conversely, when drag racing you want to hold the RPM's not only on initial lauch, but at each shift point. The heavier flywheel/clutch combos assist in that effort by, as several folks have pointed out, storing energy that can cushion the RPM drop both the at the initial launch and again at each shift point.

One of the quickest ways to drop ET's in an otherwise stock vehicle is adding in a heavier flywheel/clutch combo. It used to be a tech inspection point in the stock classes in NHRA but it's been awhile since I've raced so I don't know if it still is.

Bill