With upcoming 2.8 api changes, addons might deeply change.
Having opportunity to handle blender (api) version change on file load in addons might be critical for long time support and could prevent further version checks in addons code.
Ideal app handler signature might pass version of file loaded.
There is a file_load handler, but it will trigger at any load and running upgrade scripts could slow down overall file loading.
This is where an update handler will be handy.
Also i’m not aware of any way to retrieve original file version loaded using it.