I don’t know about dogs but you can pretty easily tell when a cat is neutered and they are much smaller. Why do dogs need a mark? I get it if it’s on strays but not on pets
I think it's mainly for vets so they know straight away if the dog is desexed. Sometimes medical issues can make it tricky to know, like humans the faster information is known in an emergency the better.
Edit: Female dogs also get desexed which you usually can't tell without the mark.
Edit: Female dogs also get desexed which you usually can't tell without the mark.