Yes. There are definitely occasions when you want to muzzle a horse. Here they are:
1. If dealing with a chronic biter. Some horses will take a chunk out of anyone who goes near them and don't seem to want to train out of it - in that case you might put a muzzle on them for grooming, etc.
2. To reduce/slow grass consumption when turning out a horse that's inclined to end up overweight.
3. To prevent cribbing.
4. To prevent eating when being ridden, especially on the trail. This is often seen when horses and mules (mules can be even worse) are being trail ridden by inexperienced riders who don't have the skill to stop them from stopping to graze.