Keeps dirt off my mouthpiece!
I use this with the CamelBak Hydrolink Filter Adapter Kit http://www.backcountry.com/camelbak-hydrolink-filter-adapter-kit#review_200160398 and it works great! It keeps the dirt and yukky canyon water off my mouthpiece when I'm canyoneering. It can be a complete pain in the butt to put on if you don't know the secret!
Use some adult lubes to easily slide the hose into the mouthpiece. You know how its done! ;)