Why advertise a PPC site on the same PPC site?

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From the side panel of my blog at lordwarwizard.blogspot.com, you can see that I have advertised some PPC sites as a viable sideline. From Wikipedia, Pay Per Click (PPC) is an advertising model used on search engines, advertising networks, and content websites/blogs, where advertisers only pay when a user actually clicks on an ad to visit the advertiser's website. To many people, the money to be made from these sites is laughingly negligible. Yet to some, I know every cent counts. Recently, I have been observing a weird kind of ads. There is nothing really "wrong" with these ads except that they are all advertising a particular PPC site on the same PPC site platform. I cannot understand why people will do this. For instance, why advertise bux.to ads within bux.to itself? For someone who has already joined as a member, he will first login then browse the ads. While logged in and clicking, he comes across an ad asking him to join bux.to... hmm, what is the whole point of this? If they hope to snare the ones who are already members but forget to login before clicking, would they be so stupid to register all over again when they already have accounts? Any thoughts on this? :P