Not because of exercise, cardio, or balance and coordination..
But because a skateboard will instantly give you a status report on the condition of your body and mind. I'm a big shitbag these days, and as soon as I step on my board in the morning I can tell exactly how fucked up I am.
If I start skating and can barely push, if I'm wobbly and weak, I know I'm way too fucked up and I gotta pull myself together. If I can still push strong, ollie, etc.. then I'm still keeping it together. As long as you keep skating you'll survive, but the day you can't step on the board, that's the day you start dying!