I think the purpose of these sites are to lower your ranking in the SEs, in addition to getting YOUR visitors.
No, it isn’t. The purpose is to generate a huge quantity of content very quickly by scraping either Google or Yahoo results. There’s no intent to lower your rankings. In fact, the use of no index,no follow means that the extra link popularity these sites might generate for your site is negated.
All they then do is add a couple of affiliate links to the top of the scraped results and trust that a large majority of the traffic to these pages goes to the sites that will make them money.