Google pagerank gives a numerical weightage to each element of a hyperlinked set of documents by analysing a link on the web page.
Google ranks each page just to show the value of that page for the visitors/users. The use of PR is that it will made the visitors/users to trust your site with it's value, so that it will increase your credibility.
I think you are asking google rank. Google page rank is totally different thing. GPR is a measurement of value given to your site and google rank is the position where your site dwells through one specific keyword.