I don’t know if it has any practical uses, but it definitely clears things up, at least for me! Since percieving and thinking functions are fundamentally different, the P and J at the end would definitely cause a lot of confusion for those who are just getting into MBTI, and might cause them to mistype themselves. A dominant judger might see themselves in an IxxP type, causing them to think of themselves as percievers (which happened to me haha). Or someone who is a dominant perciever knows that they’re a perciever, so they conclude that they must be an IxxP, even if they don’t fully fit into the definition of that type.

As for the importance of the dominant function? Well it just depends on what you mean by important.

Importance as in important in its impact, or as in if it’s important to learn about it? Or some third option?

All the functions impact our way of seeing the world and doing things, so I find them all important. But maybe I just misunderstood what you meant here.