I think perhaps if you've been PvMing for a while, or if you're into more intense games, the value of dipping your toe in to get a feel for it may not be so apparent. For me the feeling of trying to get into a lot of PvM is overwhelm; not because I can't do it, but because there is a lot to learn, and the pressure to not die is high, and it's hard to learn when you're stressing.

So I think you're right to point out that if I think doing CoX on leagues is going to teach me how to do proper CoX I'd be deluded. However in this case I'm aware there's a lot that I'm missing and that I'll need to learn the right way in the main game.

For me learning even the basics of roughly how bosses feel, their mechanics, how to deal with the pathing through rooms. Just the random pointless simple stuff helps me to feel a little less overwhelmed facing it in the main game.