As a new cyclist of mediocre abilities (20 miles at 10-12 mph on MUP's) the best reason is mind cleansing.
Those of you who have been on a saddle for a while can do this automatically, much like we do driving. But cycling is not quite automatic for me yet, so there are lots of small tasks that need to be done, need to pay attention to the path (evil squirrels almost took out my wife last week), traffic, weather, hydration, and a zillion other things.
In other words, take your mind out of work, family, money, or the Chicago Cubs and focus it all on getting there and back. Amazing!