How important is a Page Rank?

For a paid blogger like me, having a Page Rank is very important because most of the advertisers usually base the number of tasks and payment for these tasks on the page rank or PR of your website.  I have no idea how to get a PR though.  To me, it's like based on luck.  I lost PR on my blogs many times, they come and go, and i don't know where PR are really based from.  Is it quality of posts?  Definitely not!  Writing quality and interesting posts absolutely helps because you gain more visitors or hits if readers find your site interesting.  

I always hear that it is mostly based on links - which means that other sites linked you up with their sites, especially if their sites have high PR.  What i don't understand is:  there are sites that are newly-created, no hits, no traffic, very few nonsense posts but they get high PR.  On the other hand, there are sites that contain quality and interesting posts, have links and lots of traffic, but with low PR.  How do you explain that?  I guess that's how it is!