Recently I had the pleasure of doing a podcast with Kim Krause Berg. It was a great chat, primarily focused on usability issues with web sites.
It reminded me though of the strong link between usability and SEO. For example, clear easy to use navigation is one of the things we talked about during the podcast. This is also something that helps the flow of page rank through a site from an SEO perspective.
For example, one problem I have seen with web sites is that publish some content on a regular basis (daily, weekly, or monthly), and when they first publish it, the articles are linked to from the home page (for reference, we will call this first group of articles "Batch 1". Then the next batch comes out, and the Batch 1 content moves to a location one click from the home page. So far not a big deal.
However, when the batch after that comes out, Batch 1 now gets pushed yet another click from the home page. In fact, over time the content gets pushed further and further down. When Batch 21 comes out Batch 1 is 20 clicks from the home page.
This is bad usability because no user will ever find that content. It's also bad SEO, because no crawler is going to go down that many clicks from the home page to find that content. The same holds true for many other aspects of usability.
In addition, usability addresses that other major concern of web site owners - conversion. For many web site owners, it just might be easier to double their conversion than doubling their traffic - yet the both bring the same benefits to the bottom line performance of the web site.