What is the purpose of textbooks? I as a student have done various courses at my university and for each of these courses a textbook was prescribed. On the other hand, the instructor provided his notes as well. The textbook was huge (approximately 500 pages) whereas the lecture notes were only 100 pages and really short and clean. During my studies, I rarely used the textbook but relied on the notes and I think that I just did fine. I am not the only student who rarely uses prescribed textbook but other students also think the same in my classes. The students that I talked to prefer notes mainly because


textbooks are expensive so not everyone can afford buying 5 textbooks each academic session (5 subjects per session) which makes it 10 textbooks per year and the average price for each textbook is $150;
textbooks are huge (about 500 pages) so students prefer to use notes which is clean, short, and summarized;
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 given the stress and pressure that students face when they get near the exam.


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 clarification but most students really do not do this. If they struggle with a particular concept, they go to the office hours of the instructor and ask for clarification. The textbooks also have many exercises so if any student needs to do more exercises, they can uses the prescribed textbook but I have not seen any student who does this. 

What is the purpose of a prescribed textbook in a course taught by instructor and why do instructors emphasize reading it?
The question is based on several false premises and this also leads to an answer:


  […] students prefer to use notes which is[sic!] clean, short, and summarized


That's not true for many students. In fact, there are students who dislike summarized text and like textbook style.


  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 […]


Textbooks have a table of contents and often an index, so it is not at all impossible to brush up on a certain topic using a 500 pages textbook last minute (given that you know the context and some buzzwords).


  […] you can look at the textbook for clarification but most students really do not do this.


Many students do this, really.


  If they struggle with a particular concept, they go to the office hours of the instructor and ask for clarification.


Some come to office hours, some don't. For my courses, only very little students come to office hours and most study by other means - presumably some read a textbook.


  The textbooks also have many exercises so if any student needs to do more exercises, they can uses the prescribed textbook but I have not seen any student who does this. 


Look harder. There are these students who do additional exercises.

Last point: Textbooks are used by professors to create their courses. There are great textbooks for introductory courses and professors do not want to reinvent the wheel (well, a worse wheel, actually). If they rely on textbooks for their preparation, why not tell the students which books they used?

I don't know if there are courses where you really have to purchase a textbook - you may just decide to not buy it and may be fine. I prefer to announce textbooks that are available online through subscriptions of my university and in the library as a paper copy (with at least one "permanent/reference copy" that can not be borrowed) so that the students can use the book for free.