Taking notes based on the textbook obviously helps to organize and summarize info in the textbook. But if I highlighted the most important areas of the textbook, and while revising I only look at these parts, won't it have the same effect as taking textbook notes (as far as summarizing goes)? Ultimately my question is, for students who have achieved good grades by taking textbook notes, what are the benefits of doing it and why is it better than just reading the important parts of the textbook?
In addition to the benefits mentioned by fgysin, you might want to include in your notes information that is not explicitly in the textbook, for example alternative ways to think about some of the textbook's material, details that are omitted in the textbook, special cases of the facts in the book, or connections between different passages in the book.
Personally, writing something down in my own words helps my understanding tremendously. It forces me to actually, properly understand the concepts and formulate them differently.
Furthermore I often use graphical aids like drawing diagrams, graphs, etc. This is often hard to do on textbook margins due to space limitations.
Apart from the features explained by @AndreasBlass and @fgysin, there is one more advantage I would like to add concerning customised notes on a textbook.
A textbook author usually try to cover a wide range of readers. Hence what might seem sufficient to some might seem inadequate to others, yet superfluous to some others. Hence an avid reader of the subject would make concise what one is familiar with and expand more in detail on the topics that one feels difficult (probably by referring more references) by separate note-taking on the textbook that may be more efficient than highlighting right on the book itself.