Anyone who’s done any quantity of web design knows that there are often two phases to the process. The first involves creating your design using nice, clean, standards-compliant HTML and CSS, and the second involves inserting tweaks and hacks to get around the bugs and quirks of Internet Explorer.
Most web designers tend not to use IE. This is not just because of its failings; it’s often because other browsers offer designers facilities which make the development process easier; perhaps the best example is the excellent (and free) Firebug extension for Firefox.
In addition, most people of a creative or technical bent don’t use Windows; they use platforms such as Mac or Linux where IE isn’t available. But they do need to check what the sites will look like for people still using IE. So NetRenderer is a useful service – you type in a URL, pick your version of IE, and it promptly displays the image of your page under that browser.