A lot of people when they are doing search engine optimization for their website become obsessed with Pagerank. In reality Pagerank has no relevance to how well your site ranks, it is entirely possible to rank first with no Pagerank at all. That being said there is a certain value to having a high Pagerank so it isn't completely irrelevant. Since you will likely get a high Pagerank just from doing the things that you need to get your site to rank well it isn't something you need to worry too much about.
The main importance of Google Pagerank is that it determines how valuable a link from your site is going to be. Although Pagerank isn't used in determining where your site ranks in the search engines it is still valuable to have a high Pagerank. Basically Pagerank is the way Google determines how much authority your site has. The more authority that a site has the more valuable a link from that site will be. That means that having a site with a high Pagerank has value since it will make it easier to get other sites to rank well in the search engines. Since most people have more than one website this is something that has a great deal of value.
The other reason that Pagerank has value for SEO purposes is that it is calculated individually for each page on your site and not for the site as a whole. That means that you can use a high Pagerank to get new content to rank well. For example in most cases the homepage on your site will have the highest Pagerank. If you add a new article to your site targeting a new keyword you would normally link to it from your homepage. Your homepage would then pass its link juice onto this article and help it to rank well even without any back linking. If your site has enough authority you can rank well for uncompetitive keywords very easy. This is why article directories tend to rank well, the main page has a high Pagerank and it passes that juice onto all of the individual articles.
For the most part building a high Pagerank is done through the same process as getting your site to rank well. This is why you can use Pagerank when assessing the competition during your keyword research. In general if your site ranks well it will have a high Pagerank, although this isn't always true. The keys to building authority for your site are to have lots of content and lots of back links. Since these are the same things that you would do to get your site to rank well it is pretty obvious that the two go together. It generally isn't necessary to worry about your sites Pagerank, if you do the things that you need to do to get ranked well it will take care of itself.