Think of the ISBN as the primary key for looking up a specific book on ... just about anywhere.
If you want to buy your textbooks before the first day of class (which you do), you need to be able to look it up and verify you're getting the book that you want. There's always the asshole professor that assigns reading from a book on the first day of class, and you wouldn't have the time to buy it from Amazon, get it shipped, and do your homework before it would be due. Your only other option is to mooch off your classmates, which I wholeheartedly endorse.
Just to reiterate what others have said... the campus bookstore is always a gigantic fucking ripoff. Don't deal with them unless you absolutely have to. Use the internet. Buy/borrow from people who have already taken the class. Ask around if you even need to buy the book.