1) I think it's one of the reasons why Google is so accurate. While other
search engines were counting on the meta tags and eventually the number
of the links, Google started to show that quality of the backlinks is
what matters. So pagerank is important!
2) The higher your pagerank is, the deeper indexed you get.
3) PR is still used in SERPs, but it's not the most important factor
like it used to be some years ago.
4) It can give some kind of weight and authority to your site, because
not anyone can get a site to be PR6 and above.
5) You can use PR to compare your site with other sites.
6) The higher your pagerank is, the more subpages you can have indexed