There are many different reasons why preparation is an essential part of learning. Most universities follow a version of the Carnegie definition of the credit hour which assumes that you will spend 2 hours of out-of-class time doing studying and homework for every hour in class. This means that a typical three-credit course consists of three hours a week in class and six hours of work outside class. Those out of class hours and your class preparation are essential to getting a good grade.
First, you should be prepared for classes in a practical sense. This means getting a good night's sleep and eating a healthy breakfast so that you are alert and can focus. Next, you need to be prepared with whatever materials are needed for the class, such as a copy of the assigned readings, note taking materials, and drafts of written work for peer-editing sessions. If you are not prepared, you will not be able to participate effectively in the class.
Next, on a university level, you must complete the assigned readings before class. Professors will not summarize for students who neglected to do the readings as that wastes the time of students who did prepare. Also, you won't be able to ask questions about parts of the readings you had difficulty understanding if you did not do the readings, nor will you be able to participate in class discussion, something particularly important in seminars.
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.