Even without a documented API, there is often one at play using websites. Example: Website is available at "http://example.com/books/1234". When loading it, you see that it fires a request to "http://example.com/api/booksdata/1234" to load the data that popluates the page. So now you don't have to use a slow browser that loads everything, but can just use your normal http client (for all the ids you know).