mraid.open should be used for click-throughs as a substitute for an <a> anchor tag. When a container encounters mraid.open it presents the URL in a browse-able window with back/forward/refresh navigation like an in-app mini-browser.
mraid.expand should be used to show the ad in a full-screen presentation. Some vendors build the full-screen presentation as a separate asset. In this case, they can use mraid.expand(url) to present the second part of their ad full screen but without the mini-browser controls.
Thanks,
-Nathan