Reasons to muzzle-train your non-violent dog
Muzzles aren't some sort of torture or punishment device, nor do they force the animal's mouth to remain completely shut at all times. They also don't stop barking, growling, or drooling. They're tools to help keep you, others, and your dog safe. With the right muzzle, your dog will still have full range of motion, and be able to eat and drink with ease. Here's my list:
Insurance: If you have a dog breed that's stereotyped as violent and aggressive, even if your dog is the sweetest thing in the world, making sure to muzzle train it, and ideally have it wear the muzzle whenever it's on walks and such, means that people can't accuse your animal of biting them. How could it? It had the muzzle on. What did the accuser do, shove their fingers into the mouth of a clearly muzzled animal?
Veterinary care: Even the best-behaved animal can get stressed at a vet clinic, and stressed animals are unpredictable animals. Even if you're absolutely certain your dog won't bite, having it wear a muzzle at the vet will provide extra security and make it easier to perform general wellness checks and other medical procedures. It also puts less stress on the vets and vet techs. (This applies to groomers as well!!)
Keeping others away: If you're out walking your dog, chances are that people are going to want to interact with it. And hey, that's fine! But maybe you're uncomfortable with that. Maybe your dog is terrified of these interactions. If your dog is a service animal, then it gets even more complicated, and actively dangerous, because others are distracting your dog from its job. If the animal has a muzzle on, people might decide that it's not worth it to go up to the dog in the first place, and assume it's unfriendly.
Future Issues: No matter how nice and sweet and gentle your dog is now, future trauma, injury, and neurodegenerative disorders are always possible, and can change your pet's response to stressors. This can make your pet far more aggressive than it naturally is. But if it's already tolerant of muzzles, then that will be extremely helpful in managing the behavior.
Self-Injury and Destructive Behavior: Even if your dog doesn't bite others, it can bite itself out of stress or compulsion, and also chew things you really don't need destroyed. Muzzling can keep dogs from engaging in a lot of destructive behaviors, just because it can't bite at things. It's not exactly an alternative to using a surgical cone, though; a dog can still rip out stitches with a muzzle, even if it can't chew at them.
Eating: Dogs eat all sorts of things they shouldn't. Depending on the muzzle and situation, you might opt for a closed-type muzzle, which doesn't obstruct breathing, but doesn't allow it to eat. Even with open-type muzzles, there's a limit to the kinds of things they can fit through the wires. Kibble and wet food? Yes. Grass, poop, twigs, small rocks, pieces of candy, marbles? Also yes. An animal, stick, wrought-iron fence post, goose down comforter? No.
If anyone has more to put here, then please add!! There's a ton of reasons to muzzle train a non-violent dog, and I'm sure I've only scratched the service. Muzzles aren't abusive! It's like crate training and leash training. They're just good skills for your dog to have.