(Please click through to Think Geek. They might be having a sale!)
Sleeping bags are designed to keep all your parts together to conserve heat. The trouble with this is that, well - it's a bag. If you have to get up to do something in the middle of the night - rescue a droid from a Teedo, smuggle the plans to the Death Star, hack up your Tauntaun, you're basically back at Field Day doing a sack race around your campsite. Most of us don't actually need that heat, so why not exchange it for mobility? That's where the concept for the Selk'bag comes in...