Someone who is a beginner will be working on easy material with a textbook that has very mild expectations of "mathematical maturity." Textbooks mostly (with a few intentional exceptions) calibrate the mathematical maturity they require according to how much work a student will have done in order to be ready to tackle the book's subject matter. Some people might think they can be ready to do problem sets in an advanced math textbook while skipping the work that would have given them the maturity it requires, but they are kidding themselves. They can't advance on the spectator track and then make a horizontal move over to actually doing the math. There's nothing wrong with people passively reading the material if they enjoy it, but if they want to tackle the problems, they are going to have to work through the earlier material by solving problems as well, and if they do that, they'll find that their sophistication increases along with the demands on it.