It replaces the search box on the Start menu, which you might be used to using in Windows 7 or Windows Vista. With the Search charm, there's room for more results and more info about each result.
You can also use the Search charm to search within an app that's installed on your PC. For example, if you're writing a paper in a word-processing app, you can use the Search charm to find another document on your PC or quickly look up something in an encyclopedia app.