And the computer keeps track of the referrals, so there is no need to manually negotiate complex deals or have other overhead that normally makes it expensive to have small partners. Every extra user interface element is one more thing for users to look at, so links should only be included when they in fact add value to the user. Also, there is a temptation to link to things that you cannot fully recommend simply because you get paid to do so. But even though strategic considerations dictate prudent use of linking, even for paid links, the person writing any individual page may still be led into temptation. Some categories of products and services lend themselves more to affiliates programs than others.
Books are the prime example because there are books about all special interest areas.
Thus every site will have some books to recommend within its area of expertise. And there is value-add from the recommendations since the specialized site will know more about the books in its field than the more generic advice possible at a general site. You can't find the best one at the bookstore site, but a travel site could have a shortlist of the best 3-4 guidebooks for different kinds of travelers. Unfortunately, most categories would only work with a more targeted set of affiliates. Take, for example, plumbing supplies.