Some individuals prescribed cpap or bipap machines to treat their obstructive sleep apnea experience reduced benefits from their therapy because they are mouth breathers.  When the individual falls asleep, his or her mouth opens up, and much of the air being pumped into his or her nose escapes through the open mouth.  It makes your mouth and nose really dry, reduces the poitive airway pressure you need, and creates a weird, constant snoring sound from you mouth. 

Lots of different kinds of chin strap are sold on ebay and elsewhere to help with this.  They just hold your mouth shut while you sleep.  I tried one (Cardinal Health's Ruby chin strap, also called PureSom.  It's the same as the Tiara Topaz chin strap) .

I think it helps.  I really can't address the other kinds of chin straps because I've not tried them.  Some are less expensive than the one I chose.   Wearing it is no less comfortable than wearing the mask alone, and it seems to stay positioned through the night.  I used to wake up during the night while using my bipap with my mouth uncomfortablly dry or with air gushing out of my open mouth.  With my chin strap, I don't.

It does seem like the model I chose is alot more snug than it needs to be, though, even at its' loosest fit adjustment.  Maybe it will stretch and loosen up with time.