what’s the point of taking art classes?

The point is the same as taking classes for any other skill, from baseball to carpentry: you have to learn technique before you can engrain the skill through practice. Some people can pick it up on their own if they’re motivated enough, by studying other people’s art, watching artists working, reading books, etc., but it’s more difficullt and time-consuming without an instructor’s feedback. Sometimes they even figure it out wrong, and develop a very difficult and time-consuming method of doing something when a much simpler one exists.

So it’s optimal to both have the classes and do extensive practice outside of them. One is not a substitute for the other.