It's not like nature isn't all around us, but we don't notice it so much - I spend close to 8+ hours a day on the computer, occasionally I'll spend an hour or so outside, but only because i'm a freelancer. So adults need that time and space, and natural settings are the perfect environment to relax and recharge...
And particularly for kids, I think it's so important...here's a story on the CBC, a book by Richard Louv on the matter:
http://www.cbc.ca/news/background/exercise_fitness/nature-therapy.html
Photo Source: Flickr: http://flickr.com/photos/kishimi/1286063308/