Let's keep it short today. The cinch back was a widely used up until the Second World War. It was used instead of a belt to tighten the waist on pants, hence the lack of belt loops on early jeans. Belt loops were added by Levis in the 1920's, but the cinch back remained for another good 20 years, until it was scrapped.
The cinch back seemed like an archaic and slightly boring detail, but after having read the book "Jeans of the Old West", I got a totally different perspective on that detail. Now, I still find it quite costumey, but I now find it quite inspiring and I may just get a pair of jeans with a cinch back one day.
Some time ago I discovered, that the author of "Jeans of the Old West" and denim hunter/collector, Michael Harris has an amazing Flickr account. You should check it out here - and by "check it out", I mean, spend some hours soaking in the stunning images - there are small movie clips too.