For one, it helps site developers get a sense of how many folks are viewing pages using Flash-free systems, thus possibly encouraging them to publish content that doesn't rely on Flash. The underlying logic of such an approach is generally twofold. His add-on trick is to enable the Develop menu in Safari and use System Preferences » Keyboard to map a shortcut for taking a page in Safari and opening it in Chrome. For instances when that didn't suffice, he would load the page in Google Chrome, a browser which has the Flash plug-in embedded within it. Put simply, Gruber removed Flash from its default location, thus trying to force sites to load HTML5 versions of content.
While recent versions of the MacBook Air ship without Adobe Flash installed, the idea of voluntarily removing Flash from /Library/Internet Plug-Ins got some attention when John Gruber wrote about his technique.