This add-on serves as the foundation for our XenForo theme product-line. It is the necessary functionality that allows us to extend the XenForo software in order to be able do things that the theme item alone cannot.
It is kept separate as it is not tied to the theme, view, images, files, or templates in any way. The add-on may not always need updating when the theme does, as themes typically need updating for every update XenForo software does