Scrapes out the badness
If you've ever wondered why you water tastes funny, its because you haven't bought one of these.
I don't know what you're supposed to do with the big brushy bit, but the small brush on the end of the wire is the ONLY way to get the mildew out of your drinking hose. Soaking in a cleaning solution like Milton just wont get it out. You will see the difference in before and after, and in what flops out into the sink during cleaning.
You could alternatively remember to properly dry and store your bladder and tube after each use, but let's be honest - you're not going to. And it only has to sit under a pile of clothes in your room once for life to develop. LIFE FINDS A WAY