Regarding “convincing someone to buy your product is not that easy”, I do agree with you, but at the same time, I remember all those products I’ve bought online. I did not search on Google for those products neither I went to the company websites to get information and buy. They were affiliate people and their blogs that inspired to buy them. PrettyLink Pro, SEO Pressor was recommended by Kimberley, I bought Profits Theme, WP Zon builder because Alex Whaley recommended. Similarly I bought CommentLuv Premium because Ileane, Kavita, Steve and many others were talking good about it.
Win-win-win. The advertiser wins because they only pay when a purchase is made (as opposed to the shotgun approach of paying to advertise to the masses and waiting for a small percentage to actually buy). The affiliate wins because they make money while providing helpful advice. The customer wins because they get a trusted recommendation for something they might not otherwise have known about.
High ranking metrics will result in getting your site on the top position of several charts such as Top Alexa sites, Top WOT sites and so more. People usually go through these lists to discover new sites and hence you will be getting referral traffic as well. Who knows? Some of those traffic may be your next big client or loyal reader of your blog.