Visitor behaviour
From a general best practice viewpoint, it is good to ensure your site's visitors are kept engaged with the site's content when they arrive at it.
Recent research and analysis also suggests that your site visitors' behaviour can have an influence on your ranking in the SERPs, in 2 ways.
1) The popularity of various SERPs results in terms of being clicked on for particular search phrases may be used by the search engines to "rank" sites. (This procedure is similar to that used by Google AdWords to rank its sponsored listings, and is also being adopted by Yahoo Search Marketing - formerly Overture - for its sponsored listings in the last quarter of 2006).
This is where the
2) Google and the other major search engines can analyse whether a visitor spends time on your site once it has clicked through from the SERPs.
The idea here is that, if someone clicks on your listing in the SERPs, then clicks the "Back" button within a few seconds, it indicates to the search engines that your site wasn't especially relevant to the visitor for the search they performed.
Thus your site may be downgraded in the rankings.
Engaging, quality site content is one of the keys to ensuring the visitors stay on your site once they've arrived at it.