Not Just for Dangerous Dogs
You should be suspicious when someone says, "ALL dogs should..."
And yet, I'm going to do that anyway.
All dogs should be muzzle trained.
What? Your friendly dog doesn't bite? Loves everyone? Loves every dog? That's fantastic! Same with my Juno. And yet, when she hurt her foot about six months ago, I took her to the vet for a check up. When he touched her leg "just so" it hurt and she let him know it with a snap. Now, she didn't land the bite as she could have - it was just a warning to say, "HEY THAT HURT." The vet was great. He actually apologized to Juno, gave her some treats, and they made up.
Knowing that he needed to continue his exam, I suggested a muzzle since she's very comfortable wearing one. That way, he could proceed (still cautiously so as not to hurt her unnecessarily) without fear of getting injured himself.
See, Juno and Joker are comfortable wearing muzzles for moments like this. I don't want them to be additionally stressed on top of being in pain. Fortunately, this sort of situation doesn't happen often.
But we're ready.
Other dogs benefit from having a muzzle if they have a history of biting - they can go out into the world instead of being abandoned in the back yard without opportunities to explore the world. Some dogs are required to wear muzzles due to local laws. Some dogs have medical issues like Pica or Coprophagia and wearing a muzzle helps manage those conditions.
Training your dog to like their muzzle isn't that difficult. There's a great course that you can use to do this yourself. I encourage you to take a look at this course (and the many others that are available at that website).
p.s. Other things I think ALL dogs should learn…
A solid recall
Not to bolt out of doors if you live near a busy street
Toothbrushing (you can add years to your dog’s life by brushing their teeth)