Horse riders use western type bits for the same reason they use spurs. Bits and spurs are used to draw the horse’s attention to a command from the rider and to get it to respond promptly to that command.
You apply pressure through a bit to a horse’s mouth. You hold that pressure just long for the horse to begin responding, then you release it. I keep telling other riders, especially novices, that the response will be faster if the release is quicker. You don’t want to unnecessarily cause discomfort to your horse.
I take some pains over teaching a horse about responding to pressure with extended ground training using a rope halter before I put a snaffle bit or western bit on him. I subject him to pressure on either side and teach him to bend (also known as flexion). I also teach him to yield his hindquarters, to simulate a rider’s one rein stop as he gets into the saddle.
I have a very effective method, which has worked very well for me, with horses that I feel are mature enough to start. I turn the horse out in a pen, preferably round, after putting a side pull on him with a snaffle on it. I keep repeating this procedure over a spell of a couple of days till I know they have become used to it. This way, I get them accustomed to a bridle and the bit without having them blow a gasket every time. The idea is to get them to accept the bridle and bit as playthings, instead of work items or items that may cause them harm.
Once the horse is comfortable, I repeat all the ground work I had done earlier with the rope halter, except that the halter is replaced by the snaffle bit. I make sure the snaffle is of the smooth western type. The snaffle brings pressure to bear on different end points: you are now applying pressure to either side of the cheek, the bars in the mouth and the tongue instead of the nose. You must be very cautious about staying extremely light with this procedure.
This way, I achieve my intention of getting my horse used to the idea that I can use light bit pressure to communicate with him. I take things easy, a step at a time, and reward him for every positive response to get his confidence up and let him know he is doing fine.