If I attend all the lectures I don't really need to study, but it is what ensures that I get an A or high B. There is simply much more information to remember compared to high school, at least for me it becomes too much because my memory quite frankly sucks. I'm sure there are some people out there that don't need to study, while others are extremely reliant on it. Studying is especially crucial if you don't understand something from the lecture, which happens more frequently at uni than it ever did in high school for me. I legit had to go to the prof twice about the same relatively basic set of definitions because I didn't understand it, then I went and thought I understood it, then I realized I still didn't quite grasp it so I went back and felt dumb as hell lol. Better to swallow your ego and feel dumb than it is to be dumb and get a worse grade, though.