The advice is/can be used in a ton of different situations, you seem to only think it’s for a very narrow and small part of those. And even then, it’s not always the best thing, someone can be themselves 99.99% of the time but sometimes it’s better to hold it back. Even something as simple as if they’re meeting your parents for the first time and you know there will be problems if you don’t tell them to not mention something, act like they think differently about something etc.

This very much isn’t just about a possible partner for you and all the time. I don’t understand how you thought that was the only thing. It’s like if someone said a common feature of cars might not be worth keeping on all the time and you read that as only a specific line of Volvo cars, in very specific situations and like you can’t turn it on and off. See how weird and dumb that is?