Relevance is of absolute importance, but probably not in the way you understand it. You can have all the backlinks you want with all the PR you want and it will never help you rank until after you channel that PR through a relevant link.
Many people seem to think that relevance has something to do with website topics. Website topics are not ever considered by search engines. Search engines are much more granular in their approach to relevance. They not only look at page topics for relevance, but they can also even look at individual page elements to determine relevance for document sections or even relevance of individual external links.
Search engine optimization is often broken down into on-page optimization and off-page optimization. Many folks have the mistaken impression that off-page optimization is simply creating backlinks. Technically speaking, creating backlinks is a promotional activity that may, or may not include optimization. Just as elements of your own pages can be optimized, or not, elements of the backlinks you place can be optimized, or not. This is what is really meant by off-page optimization.
Many places where you place your backlinks allow, or even encourage, you to create unique and relevant content on the page where you place your link. This helps their users to understand what they can expect to find when they follow the link. Search engines look for those same signals of relevance and use them to score the landing page’s relevancy for specific keywords. PR is simply used to weigh the total value of that particular relevancy signal.