PageRank is one factor among many in the algorithm. It also has something to do with how deep Google will crawl your site. There is some value in having more PR, but...
The PR you see is toolbar PR (TBPR) and it's not the PageRank Google us using to calculate anything. It's also not something you should overly concern yourself with. Lower PR pages outrank higher PR pages all the time.
If you build links where it makes sense to build them then your PR will go up. The trap people fall into is in thinking that PR is the end goal, when it's not. Chasing after PR for the sake of PR isn't going to lead to a successful site.
It's like trying to win a race where to get to the finish line you need to pass three different stages en route. Someone somewhere observes that all the previous winners were among the top 5 people to reach stage 3. So you decide that you need to be one of the top 5 to reach stage 3 in the next race. But in your rush to complete the first 3 stages you forget to pace yourself and run out of energy before you cross the finish line and finish last.
Stage 3 wasn't the goal. The finish line was the goal. PageRank is not the goal either of SEO. Targeted traffic is the goal of SEO. You can get that with or without PR. If you focus on getting targeted traffic, say by getting links on pages where people might also be interested in your page, then your PR will end up going up. But your PR going up doesn't necessarily lead to targeted traffic.