The purpose of the AOI in Coastal Vulnerability is to designate the area of coastline that should be analyzed for exposure risk. Since the exposure score is a relative measure, scores at one shore point are dependent on values at other shore points. For that reason users might want to exclude certain areas (e.g. small uninhabited islands) from the analysis. They can use the AOI to do this, with one exception:
The AOI extent is also used in the surge exposure algorithm to clip the continental shelf contour, prior to distance measurements. This means users might be forced to create an AOI like this one (from Jess), which needs to extend far north to capture the important shelf contour, but then makes it annoyingly hard to exclude tiny little islands on the north shore from the AOI.
There are reasonable ways to process a global continental shelf input layer other than using the AOI as a clipping box.