Sure, you usually don't want that feature in private setting, but you almost always want that feature in a commercial setting, and lots of communication happens in that context.
E.g. vendor-customer helpdesk chat, internal workplace communication including "less internal" things like different subsidiaries of international companies, etc, etc. Half of financial world runs on Thompson Reuters messenger which is essentially glorified chat. What if your boss sends you a message "hey, do that risky thing right now" - do you want that (likely informal) means of communication to have deniability? Does the company want deniability in the app in which random middle-managers message their subordinates? It makes sense for companies to mandate that teams choose only communications platforms that support authentication and nonrepudiation.
As soon as money, any kind of disputes, and the smallest chance for future legal proceedings are involved, anonymity and deniability are flaws and not features - as I said above, superficially similar use cases can have opposing and incompatible requirements.
Even going back to the commonly discussed use case of Signal for journalism. Let's say a journalist interviews a whistleblower over a mobile messaging app - you'd want anonymity and deniability there. And five minutes later that same journalist asks a clarifying question to the official head of that agency, likely also using a mobile messaging app, possibly the same one. Do you want the answer of that official to have deniability, or do you want that journalist to be able to cryptographically prove that the official lied?
