Why use a bitless bridle......?  

Bitless bridles can provide a simple, non-invasive, means of communicating with the horse to allow connection and freedom.    Most people notice an instant change in their horses' performance and demeanor when switched to a bitless bridle.

Numerous physiological and psychological problems found in the horse can be linked to the bit due to its interference with normal breathing and its infliction of pain.

In short, the horse is a nose breather and is designed to have its lips tightly sealed when exercising.  Anything that breaks the mouth seal (i.e. - a bit) starts a process of neurological conflict in the horse.  

Why?

When the mouth is open, the brain signals the parasympathetic nervous system into action, which is eat, rest, relax and digest.  When the horse is running, exercising, or exerting itself the mouth is naturally sealed shut and the sympathetic nervous system is stimulated into action.  People are unknowingly creating this conflict when they put a bit into the horses' mouth and ask it to exercise.  

The bit breaks the mouth seal, stimulating the parasympathetic system (eat, rest, digest) and yet the rider is asking the horse to exercise which is the OPPOSITE, sympathetic system.  It is a conflict of the nervous systems which can be linked as the starting point for literally hundreds of problems/diseases in the horse