How does Primordial work with all the dialects?

In dnd-5e, there is a language called primordial, which is the language of the elements. The PHB says it is spoken by elementals, but most elementals actually don’t speak it. They speak “Auran”, “Terran”, “Ignan”, or “Aquan”, languages that are not listed in the PHB. In fact, I don’t think any monsters speak primordial. XGtE says the following, under the Storm Sorcery sorcerer subclass:

You can speak, read and right primordial. Knowing this language allows you to understand and be understood by those who speak it’s dialects: Aquan, Auran, Ignan and Terran.

Does knowing primordial automatically allow you to understand and be understood by all the dialects? Or do you have to be a storm sorcerer? If not, then isn’t the language primordial basically useless, as no one speaks it?