Treeless saddles are very comfortable saddles. They are light weight and easy to swing. Also they are not very good to use when coming to speed and timed events, because you may slid in the saddle. But they are a very good saddle.
These saddles typically feature lightweight fiberglass pommels and cantles, with leather seats and foam padding. There are no other rigid structures forcing the saddle to keep its shape. There is one of a growing number of riders who have begun using treeless saddles on their horse, convinced that they provide greater comfort for the horse and enhance the rider’s sense of connection.
I think, the treeless saddles seek to provide more flexibility and allow the horse to move more freely, and some riders feel they work well. Me I'm not at one hundred percent for treeless saddle, but I am not closed on this subject.