The illustration above shows what happens when a snaffle bridle is used without a throatlatch. When the reins are pulled, the bit is pulled up in the horses mouth causing the cheek pieces of the bridle to bow out.
This can cause the bridle to have enough play to slide over the ears of the horse and fall off, especially if the horses head is lowered or he goes to shaking his head.
A properly adjusted throatlatch will prevent this from happening. There should be no more than a few fingers width between the horse’s jaw and the throatlatch. If the throatlatch is too loose it will not be any better than not having one at all.
However, be cautious not to over tighten a throatlatch, because when a horse flexes or tucks his head, a too tight of throatlatch could compromise air supply.
There are times where you might want to use a bridle with no throatlatch, like in the show ring. In this case, you can tie a small string from buckle to buckle, under the jaw.