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.
I have "met" people who are both blind and deaf. They use sign language by putting their hand on the hand of the signer and follow that way. (I only know a couple signs so it wasn't worth trying to talk to them except via interpreter and so saying I've met them is a bit strong)
This also is proof that sign language works in the dark.
ASL (and sign language generally) and tactile sign are two different languages, and language types that are at least as different as verbal languages and seen/sign languages - an American deafblind person who uses tactile sign and meets someone who uses ASL isn't necessarily going to be able to communicate with them. The signs and underlying language structure (morphology/syntax/etc.) are all different. I wouldn't consider the existence of tactile sign to mean that sign language works in the dark because they're two different language types, but the terminology is confusing and I have a linguistics background so I don't know if the author was including tactile sign or not. I'd guess not since it's about teaching his daughter who is d/Deaf, not deafblind, and tactile sign isn't in much use outside of the deafblind community. d/Deaf and blind people don't use it much, I believe.
They can definitely be distinct languages as is the case with protactile. For the deaf-blind people I've met, ASL is their first language and protactile is more of a second. Maybe it's different in other areas of the world.
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.
This also is proof that sign language works in the dark.