Sorry for the long answer, but the title of the question scares me, and brings visions of a world where few (nobody?) value textbooks anymore (the horror!). So I knew I had to jump in. This is my (a student's) perspective on why textbooks are important in a course.
textbooks are expensive [...]
As a computer science undergrad from India, I do sometimes feel a pinch in my pocket while purchasing particularly expensive textbooks. This rarely happens, however, since I view the money spent on textbooks as an investment for my education – much like the tuition fee for my university. Moreover, many authors (and sometimes universities) generously provide free PDF copies of textbooks online for personal use; for example Computer Vision, Deep Learning.
The textbook was huge (approximately 500 pages) whereas the lecture notes
were only 100 pages and really short and clean.
There are a couple of very simple reasons for this. Firstly, A (good) textbook is written in a manner in which new material is introduced gently, giving background information and building the idea from the ground up which makes the concept seem simple and intuitive. For instance, in our course on compilers, we use the legendary Dragon Book, which when read judiciously builds up complex ideas from scratch in a manner that makes them seem trivial, when they are obviously not. (To appreciate this – if you have done computer programming before, and find it hard, consider how hard it must be to write a program that can correctly decode a general computer program to make the computer understand what to do).
Secondly, I don't know which course you study, but if it is something that can have a strong mathematical background (such as CS), then I doubt if your instructor's notes will do justice to the material in terms of elucidating the underlying proofs, analyses, etc. In a subject like CS where these aspects are crucial parts of the material, omitting them for the sake of simplicity doesn't seem like a good idea. The notes are an aid to learning, they are not a replacement for the textbook.
During my studies, I rarely used the textbook but relied on the notes
and I think that I just did fine.
That is very common, unfortunately. Although I always read the textbooks for all classes I take (I love textbooks :D), I am often not among the top scorers in a course. My observation is that this is because most of the top-scoring students in a course do not study the material, they study for tests. They restrict themselves to instructor's notes and test questions from previous test papers to prepare themselves for tackling the test well. They do not spend time in building the ideas from basics, which is what textbooks are for.
textbooks are huge (about 500 pages) so students prefer to use notes
which is clean, short, and summarized
As Dirk mentioned, that's not really true.
It is true that during an academic session, a student studies for a
particular subject, but he may forget some of the material so he needs
to study it again few days before the exam. Studying a 500 pages
textbook few days before the exams is practically impossible [...]
As Dirk mentioned, it is not impossible to revise smartly from textbooks using the index, etc. But my personal opinion on this is that textbooks are sometimes not very efficient for revisions / studying for exams. For these, your own notes (made during lectures or while reading the textbook) or your instructor's notes can be a useful aid (not a replacement).
The only benefit of the textbooks that I can tell is that if you find
a particular concept unclear, you can look at the textbook for
That is not the only benefit of textbooks! As I mentioned earlier, (good) textbooks provide context to the material, they are responsible for building ideas, not simply providing information. In my opinion, if you really want to understand and appreciate a subject, then this deep, contextual learning by building new ideas over old ones is an important journey – and an enjoyable and fulfilling one too!
If they struggle with a particular concept, they go to the office
hours of the instructor and ask for clarification
Perhaps this is not a problem with you, but I (and many of my peers) have often suffered due to the lack of a qualified instructor for a subject. During such times, textbooks have been the guiding light for many students like us. It was during times like these when I realised the value of textbooks.
Finally, a good textbook is usually written by a person well experienced in the field, for the sole purpose of explaining the material to the reader. If you do have the opportunity to meet such an expert in person, who has the time, patience, and skill to frame his thoughts in a simple and coherent manner, then do go talk to him – but still save your textbooks because you can read them again and again over a period of years ;).