It's said to increase something or other - basically the recip is said to help the ranking and keywords on a page. Some people are linking to thehun.net and worldsex.com to which there is no logic... until you consider search engine keywords and spidering. I'm not sure on the specifics, but look at the following URL, if anyone could tell me the logic behind THIS one then I'd be forever greatful...
http://www.oldermansex.com/friends.html