But why would Internet Explorer need extensions? The first, and most obvious, answer is that Microsoft is trying to win back some enthusiasts to their browser (and its platforms). If Microsoft relaxes certification requirements for extensions, compared to Windows Store, it could also bridge the gap between native app and web app for enterprises, especially smaller businesses, a give them a platform without the burden of sideloading.
We might also see this being used by third parties to contribute to Internet Explorer development. In much the same way as Nokia experiments with WebCL by a Firefox extension, others could use Internet Explorer add-ons as a testing ground. In fact, according to their aforementioned 2008 paper, Microsoft Research already tested an OpenGL rendering stack in Xax.
We will probably find out more about the next IE soon.