Probably because if you just do what you're comfortable with, you won't learn anything new. If you're forced to do something uncomfortable, your brain learns more about your body.

Makes sense. They mention other benefits to forced exercise too. I wonder if it is an evolved response to stress. Sort of: if the bear is chasing you your body responds differently to deal with later bears.

I don't think it's that complicated. I just meant learning different ways to move your legs: get your abs involved, pump your arms differently, etc. Parkinson's can make people "forget" how to walk even if they can still run. So learning how to move your body in different ways could help combat that. https://www.nytimes.com/2010/04/01/health/01parkinsons.html