The idea behind Accelerators has always been to reduce the distance between services and the end-user. Every Accelerator does something with a service.  Therefore, there’s a verb out there that describes what the Accelerator does.  We added this verb as a field in the Accelerator XML manifest, and thereafter started referring to it as the Accelerator’s “category.” Once we had categories and default Accelerators, we had the necessary pieces for the category system we have today.  First, we present a top-level menu that contains the default Accelerators for every category the user has.  Since most users will want to use their default Accelerators a majority of the time, this really helps minimize any hunting they have to do to find the right Accelerator.

More info: IE blog