The internal linking structure of a website is very important considering that each page has a potential rank with Google and other search engines. Internal linking refers to the links present on the domain which point to other locations on the same domain. Distribution of link authority and ranking attributes is where it becomes very interesting.
As an overly simplified example, Apple.com is a PR9 website meaning it has a lot of clout in the eyes of Google.
The Apple Store on the other hand is PR7. Both ranks are extremely high, most websites never reach this point. However if Apple wanted to raise the PR rank of the Apple Store page, they could do so by linking from the PR9 home page to the Apple Store page, which is what they are doing and surely plays a major role in the current ranking of each page.
Although this is a simplified example, it should make sense that if your domain is PR5 and your post pages are PR0 and PR1 then by distributing what I and many others call ‘link juice’ from high PR pages to low PR pages internally, we are able to help increase an individual page’s PR.
This goes hand in hand with site architecture I touched on in an earlier post. The idea is to be able to redistribute the available ‘link juice’ and increase or raise ranking. So understanding that you have both on-page and off-page optimization, we need to remember that although we may not have external links pointing to each specific page or post, we still have control over our internal linking structure and that is a big plus.
There are certainly many variables affecting PR level. Some of these might include domain age, relevance, external links, and much, much more. We don't know exactly what affects it, or how much effect each item has on the whole. Generally, focusing on high quality products and content and focusing on a strong backlinks campaign will help increase relevance in the eyes of the search engines.
In the end, a properly constructed internal link structure will always help a site in terms of quality, likely ranking, and probably increase traffic. If for no other reason, a site with a strong internal linking structure will help the visitor navigate the site with ease. If a site has commercial intent, this becomes even more important because you want visitors to find what they are looking for and buy it.
Adobe.com is also a PR9 website. Both Adobe and Apple place a lot of weight on their internal linking structure.
Take a look at this video in which Matt Cutts is explaining the importance of internal links.
Summary of video: Interlinking is one of the 3 most important optimizations that every blogger should do for his site’s SEO and conversion.