Google pagerank's only a small factor in SEO, It can not compare with SEO. In other words, Google has not updated PR for a long time thus now pr is not important and exact. Don't care about PR, focus on content and traffic you're having for your website.
Although Google Pagerank's only a small factor in SEO as said in the above post, but I think it is the primary and initial thing for which everyone start doing SEO, to increase their PR. So yes they are related to each other a lot but not at great extent now as page rank is not only the factor to rank high on search engine.
PR is just a factor it can effect you up to a limit only and on the other hand search engine optimization is a process that includes the promotion of keyword you want to rank on the world wide web in the direct or indirect way and if your keyword is ranking on top through out the year then may be it can be a reason for your PR.
By checking the google page rank we can know what is our current site exposure, and what we have to do, in order to increase our exposure by doing correct seo strategies. we can increase our exposure which inturn increase our page rank.