Preparation is essential for may reasons (when it comes to anything in life, classes included).
First, one cannot take notes if they do no have the materials needed. Therefore, materials, such as writing instruments, paper, or electronic tablets (laptops/desktops included), are needed for notes. Without notes, one does not have the specific material needed to study.
Second, one needs to have the textbooks or literature required. Without these, the student cannot read the required material.
Third, if one does not read the required material, they cannot take part in an academic discussion over the material. While one can discuss something they are unfamiliar with, one learns more by examining the material a discussion will cover.
Fourth, one needs to be mentally prepared. If one comes into a class upset, the likeliness of their mind wandering is great. If one comes into a class focused upon education and the class, then the likeliness of them succeeding in the class is greater.
Lastly, being prepared leads to less stress. If one is not prepared then they may feel stressed out. On the other hand, if one has completed the assignments, read the material, and understands the material, they will be more likely to be able to focus upon the class.