Why do cats wear scarves in anime?
So they stand out from other, normal Cats.
Similar how many Protagonists in Anime have a different hair color, eye color or some other special feature, and many Girls have varying hair-colors so they contrast from their
predominantly brown / black haired normal Classmates.
So they appear more human.
By giving them human-like characteristics, like wearing accessories, they appear more civilized, intelligent and human like.
Because we do this in real-life to, I guess.
Some people just like to dress up their dogs and cats, so it wouldn't be impossible for Anime-Characters to have the same ideas?