If you are selling backlinks pr is very important because the higher the pr the more valuable the link will be. Google also crawls websites with higher pr more often.
The higher your Pr the higher your moral , some believe it's useless others believe it's held in high regard.
If you have a thirst for knowledge, then there's no better place than Wikipedia. http://en.wikipedia.org/wiki/PageRank
Kinofsanda is spot on, but it's not important in terms of ranking, it has no influence on were your site is ranked in the SERPs, it's purely a metric tool for Google to use and others. Regards, EIMG