Occasionally on TV the need arises to communicate with someone who is hearing impaired. Rather than brining in an interpreter, one of the series’ regularly characters is suddenly revealed to be fluent in American Sign Language.
Ted demonstrated it a couple of times in How I Met Your Mother.
Levitt in Barney Miller explained his fluency by saying a family member was deaf.
Howard in Big Bang Theory knows ASL, I suppose, for the same reason he claims to know seven other languages.