Mr. Garrison has often used the doll as an expression of his own repressed feelings or beliefs, including homosexuality while he was still closeted and his racist leanings when Mr. Hat was a member of the KKK. More commonly he is used as a teacher's aid via ventriloquism.
Due to Garrison's southern drawl, he pronounces the name as "Mister Hay-ut".
Mr. Garrison: I don't know, Kyle. Did you ask Mr. Hat?
Kyle: I don't want to ask Mr. Hat, I'm asking you!
Mr. Garrison: Oh I think you should ask Mr. Hat.
Kyle: Mr. Hat, may I please be excused from class?
Mr. Hat: Well, Kyle. No!! You hear me?! You go to hell! You go to hell and you die!