What is the purpose of a cutback saddle?

It isn't to keep pressure off the withers, that's silly. The cutback saddle is used for saddlebreds and for most gaited horses. It seats you further back on the horse's back so that they can move their shoulders more and step higher. Horses ridden in this saddle keep more weight on their back end because your weight is more towards their back end. This saddle can sit on the withers just as other saddles do but that is the rider's choice. Personally I put mine behind the withers.