|Web surfing is so easy that you can easily train your computer to do it for you.|
Some authors and publishers like to check the "Amazon rank" of books on a weekly basis. After once searching Amazon for a book's page, it's easy enough to bookmark that page, have a browser bring it up, and inspect the page for the "Sales Rank" value.
That's what most authors do. Suppose, though, that you don't want to wait for your browser to render everything that Amazon puts on the page, or, even more potently, that you want to feed the sales rank into your own algorithms for logging, charting, projecting, and so on. You need a computer program to extract the information that Amazon formatted and delivered for human reading.
The client-side Web programming that this requires turns out to be quite simple.