Reading boring or unfamiliar books could be helpful for readers because it would require them to pay more attention to the writing itself. This is because regularly, when you read a book, you pay a lot of attention to the plot. When you are reading a book with little to no plot or is uninteresting to you, it makes you pay more attention to the writing.
Say, for example, you are watching a movie that is uninteresting or boring, but you have to watch it. Instead of focusing more on the plot and what happens, you would focus more on the technique and the acting. It is a bit like that.