From reading this paragraph I think I now understand another reason for comments that were being left at one of my other blogs that were pure advertisements.
And so I also can see this is why many blogs require some type of sign-in to prove that I am not a machine.
"Spider food websites are intended for consumption by search engine software which spiders the web to index pages. Their purpose is to increase the perceived importance of another website by their links to it, so promoting that site in search engine rankings."
- Open Directory Editorial Guidelines - Site Selection Criteria (view on Google Sidewiki)
