Why do people ask for the ISBN code / number when they buy books ?

Just curious. I was selling some books on the internet and people always ask me what is the ISBN number for those books. if they want the details about the books, should they look for the author, the publisher, the edition or year of the book ? I always didn't ask them why but now thinking back about it, I am curious. Thanks.|||The ISBN number is unique to each book and instantly identifies it specifically. For example, suppose you are looking for "Pushing Your Finger into the Pencil Sharpener" by Iman Idiot, published in 1956. But there have been three new editions of the book since then, and good old Iman has written several other books with similar titles. If you identify the book with its ISBN number, there is no question about which edition it is, (this is particularly important with school books, which often go through several changes in edition), who the author is (if there are several books with very similar or even the same titles), and any other information which might be confusing or change over time.