This is Tony Mulqueen’s answer. I’m just being a little more abstract.
There is a popular misconception that the primary purpose of language is to communicate.
Language is a social phenomenon, practiced by social beings. And one of its primary functions is to demonstrate allegiance to the groups the speaker belongs to.
If you speak Greek, you are demonstrating allegiance to Greekdom. If you speak in a camp voice, you are demonstrating allegiance to gaydom. If you say “cuck” a lot, you are demonstrating allegiance to the alt right.
It is the same with standard forms of a language. Standard languages did not drop from the Heavens, as instruments of pure dispassionate logic. They were formed in a social context, and they demonstrate social allegiances.
Standard languages are also not native to all social groups within the speech community: they have to be learned. If they didn’t have to be learned, there wouldn’t be any need to standardise to begin with. And if you use the standard language in a social context where a different native form of the language is expected, then you are demonstrating greater allegiance to the standard language’s values, than to those of the group you are amongst.
In other words, you are behaving as a snob. And possibly a class traitor.
The main context where using the standard form of the language is appropriate (if it is not already your native language variant) is in the workplace. Hence, African Americans saying “I can speak Job Interview.” Hence also the Greek Cypriot derogatory term for Greeks from Greece being “pen pushers” (καλαμαράδες). As far as they are concerned, only a pen pusher should be speaking standard Greek.
My friend Marija’s father Ambroz arrived in Australia from Croatia in the 50s, and went straight to the nearest factory to apply for work. He’d learned standard English back home, of course. And the first thing he said in English when he got there was, “To whom should I speak to apply for work?”
Yes, standard English. Spectacularly inappropriate on an Australian factory floor.
In response to these social realities, you can rail against everyone else being peasants, and not appreciating the virtues of the linguistic standard. Or you can admit that there is no such thing as linguistic superiority, only linguistic fit-for-purpose. Ambroz was not demonstrating linguistic fit-for-purpose by using standard English where he did.
There is no such thing as linguistics superiority, but there is such a thing as social subordination. Make sure you don’t mistake one for the other.