To provide the ability for new permission models to be loaded during runtime.
At present new permission models can only be activated at bootstrap. This is because permissions.xml is only read during startup cf https://docs.alfresco.com/5.2/concepts/implserv-permiss.html
The ability to activate new permission models dynamically in the same way that new content models can be activated dynamically
This would allow new permission models to be tested and deployed more rapidly just as content models can be.