It keeps me off of the tops of tall buildings with high powered rifles...
That... and it helps me stay as active and as pain free as possible, was the reason I met my best friend (my wife) and really does keep me sane.
During my last physical my doc looked at my bloodwork and said "On the one hand, your overall cholesterol is a little high. On the other hand, your good cholesterol is off the charts and that means your cholesterol ratio is excellent. Beyond excellent, actually."
He attributed that to my riding.
Cyclists of the world, unite! You have nothing to lube but your chains!
When I started seeing my doctor 10 years ago he was checking vitals and asked if I had a heart condition as he said my resting pulse was extremely low (52)... had to tell him that I had just ridden my bike to the appointment (10 miles) and that I was probably still running a little fast.
When he asked how much I rode I said, about 10,000 miles a year.
He said that the rest of my numbers were what he wished all his nearly 40 year old patients had.
When my wife and I first started seeing each other I was 42 and she was a little shocked and surprised at some things even though she was well aware and has always been really supportive of my need to ride and she is my favourite riding companion.