When fitted properly the main purpose they serve is to stop EXCESSIVE opening of the mouth. I also find they make the face of most horses more complete when they are used.
Too many people fit them too tightly which creates tension in the horse as it can't properly mouth the bit and accept it properlly.
If you use it remember you should be able to fit 2 fingers between the noseband and your horses nose, if you didnt know :)
Nope, it shouldn't even keep their mouth shut, not if fitted properly. It's just fashion, lol. To hold the mouth shut, it would have to be low, like a dropped noseband, and TIGHT.
A regular "french" caveson is fashion only. It should be 2 fingers (about 1.5") below the cheek bone and snug, but loose enough that you can fit a finger under it easily. The horse in that ad has his a tad too high.
Here's my daughter last June, for a spring/summer photo shoot on my mare. She's wearing her "show bridle" (that never has seen any shows, lol). Her's is a tad high too, but not as bad as the photo on Dover.
(Yes, she normally wears a helmet, pants, and boots when riding! Lol)
Oh, I am just inside city limits! We are more towards the southern part though! I'll come with you to Arizona! Our weather is WAYYYYYY to strange. One minute we are all wearing coats and Uggs, the next, shorts and flip flops!
It's really just a fashion thing and people like the look of them. I don't often have the noseband on mine unless showing. In fact I only have show bridles with the noseband on. It doesn't really stop the mouth opening at all...possibly a tiny reminder to the horse that there is a slight pressure there... and to be honest I don't think makes one bit of difference to control etc.... It's just meant to look nice... though I like the look without it!!!