Reasons are many and this list will go on for sure. However to make it short, here are a few of the many ups of affiliate marketing:
An affiliate marketing program is by far one of the best advertising tools available, because it provides incentives for both the affiliate and the merchant. Affiliate marketing has grown greatly over the last couple of years because more and more webmasters are beginning to realize it is the most efficient way to handle your online advertising. It is extremely flexible and provides numerous benefits that you won't get with any other ad program.
Affiliate marketing is a type of online advertising where online merchants/advertisers pay webmasters/publishers who promote products or services on their sites for results (such as a visitor purchasing a promoted product or service). The merchants are usually referred to as affiliate merchants and the publishers are referred to as affiliates.
Benefits of affiliate marketing include the potential for making the advertising process much easier for both parties - accepting and approving registrations and orders, generating unique sales IDs, tracking of results, etc, and payment only for results - purchases, registrations, etc.
The fact that advertisers pay their publishers only when a new client is introduced makes affiliate marketing a comparatively low-risk, well-rewarded business venue for both merchants and publishers.
As you can see from this article, the affiliate marketing is a win-win situation, and that is why it is still one of the most popular forms of online advertising.