Page rank lets the users know how relevant your site is to the domain i believe!
For example if your domain is today.whatever and your content is about a tv show or birds or something, it wont be high!
Also its how much the material changes on your site, if your material changed a lot on your site it will be higher as the google bot needs to search your site more for its "book keeping"
This is relevant as if you are looking to advertise on a site, it is obviously better to have a site which is relevant to yours and that the google bot is searching it more than another, and the page rank is a good indication.
Fake page ranks are easy to detect, there are many sites to verify it, also, i have noticed, and i would like to ask the question, how come the page rank can change so dramatically when everything is running the same as before?
Eg: I had one site i was working on and it got up to
PR 5 then two weeks later, even though work was still being done on the site, new material etc, the page rank fell to just 1.. Anyone have any ideas how this could happen ?