What is Page Rank
PageRank is Google's algorithm for checking the value of a page when displaying it among search results. A higher page rank will help to push a page towards the top of search results, a lower page rank is not a negative factor except in the fact that it is not improving the site's score. Because of this, PageRank is very valuable to a domain owner, allowing the domain owner to help boost their page in the search results and therefore bring in new traffic. The basic idea behind PageRank is that the search engine will look at the number of links directed to a page and try to use these to determine how popular a site is and how relevant it is to the search.
PageRank avoids linkspam by putting judging the value and relevance of an incoming link directed to a page; a page could have a hundred incoming links coming from websites of minimal importance and relevance and still have a significantly lower PageRank than a site that has an incoming link from a page with a healthy popularity and is relevant to the linked page. The idea here is that judging a site's worth by the content that links to it will prevent sites that contribute very little to the content of a system of links will prevent spammers from bumping their pages to the top by linking several irrelevant, barely extant pages to one page that they have developed.
A good PageRank can be a great think for any domain owner. If, for example, you are trying to run an online business, a high PageRank means that your sight might be seen before your competitors. Building PageRank, however, can be a long, tedious process. This is one reason that buying an expired domain can be a great idea. It's possible through various domain services to search through expiring domains by popularity, so if you decided to invest in an expired domain that had a good PageRank it would be possible to search for one just on that criteria. Domains that have a high PageRank or other forms of SEO typically resale for a bit higher than their unranked counterparts, but the money saved from buying a domain that has already had all of the work done can be significant. The time saved by capitalizing on the work of the previous owner is also significant, and translates to less time you need to spend doctoring a website while you could be tending your business.