Neuromuscular conditioning. Trains the nerves to fire the muscles at the correct moment. Many training modalities are outside the envelope of normal riding. This is one of them. Seems to do some good for the leg muscles, too. I do this only on my rollers, only in the winter, and only for 45 minutes max. I do it as a recovery drill, keeping HR in zone 2, thus gearing low. I start at 15 minutes and gradually work up. It's fun to see that zone 2 cadence gradually come up from the training effect.

Thus I don't necessarily pedal at 120 or at least not to start with. After my fall lay-off, I might hit only 112 in zone 2 and gradually work up to 116-117. I don't think I've ever managed 120 in zone 2. Of course I could easily do that in zone 3, but that's a slightly different workout. It could replace some zone 3 interval work, though I doubt there's any greater utility in that.