They're somewhat useful for PDF files, and a few other formats, but the problem is that there aren't really many preview handlers available, as they're third party products. For example, you can install the Foxit PDF Preview Handler: http://timheuer.com/blog/archive/2008/05/09/foxit-pdf-preview-handler.aspx. Then, when you click a PDF file in the search results, it should show a preview of the PDF in the lower-right pane.

There's also a preview pack that has handlers for some code file types: https://previewhandlers.codeplex.com/