There was a time when—on the Internet—a page view was a page view. New content meant a new page. But with JavaScript and Flash, the definition of a page view can be a little bit different. If you have a slide show in Flash, isn't each slide a page? What about AJAX applications that load content in the browser without a reload? A visitor could land on an AJAX enabled site, be there for an hour browsing through content and register only one traditional page view. If you had server-based tracking, you could track each of these AJAX hits to the server. But there are still more advantages to using JavaScript to track visitors. You will get data on screen resolution, browser plugins, and other details not available in server logs. Luckily, you can manually trigger page views with Piwik, but it is up to you to determine what a page view is.
You may even have to trigger a page view when it has nothing to do with Flash or JavaScript. A good SEO practice, to concentrate...