Amazon operates a volume-based advertising fee structure. The more products that are shipped as a result of your affiliate links, the more you'll make per sale. Once you have sold enough products to move up to a different advertising rate, all subsequent sales will give you commission at that rate, until and unless you reach the next fee level. Note that some products are exempt from this commission structure.
Affiliate marketing is very appealing to some publishers as well, because it can allow them to make considerably more money than they would under an alternative monetization strategy. Though the specifics of payout arrangements can vary a bit, in general affiliate payments will be significantly larger than the revenue generated from a click under a CPC pricing arrangement (or the effective CPC under a CPM arrangement). For high margin products such as e-books, for which there are no material costs, affiliate margins can be as 50% of the total purchase price. So it’s not unheard of for affiliates to generate $100 or much more from each referral.
Affiliate marketing is commonly confused with referral marketing, as both forms of marketing use third parties to drive sales to the retailer. The two forms of marketing are differentiated, however, in how they drive sales, where affiliate marketing relies purely on financial motivations, while referral marketing relies more on trust and personal relationships.