From my understanding, they collect information in order to list things in their search engines. So, if you google search a term, then all websites with that term will come up (unless it is a search engines that truncates results). If the term is on here, then the google spider will pick it up, store it in memory, and then that is how it lists this forum in the google search when the term is searched.
That is the best way I know how to describe/ explain it. I hope it makes some sense.