Or if you have to talk to a visually impaired person. My vision impairments are on the very mild end of the spectrum, but they're still enough to prevent any effective use of or learning of sign.
This also is proof that sign language works in the dark.
What I've noticed is that in deaf-blind contexts plain ASL is terrible for back-channeling information... is the listener paying attention? Agreeing? Disagreeing? Laughing? Protactile communicates these back to the signer using touch, and not touching is considered rude.