When talking to other applications or serializing the modifier
state (and in particular when implementing a wayland compositor),
the effective modifier state alone is not sufficient, one needs
to know the base, latched and locked modifiers.

Previously one could do with backend specific functionality
such as clutter_device_manager_evdev_get_xkb_state(), but the
problem is that the internal data structures are updated as
soon as the events are fetched from the upstream source, but
the events are reported to the application some time later,
and thus the two can get out of sync.
This way, on the other hand, the information is cached in the
event, and provided to the application with the value that
was current when the event was generated.

https://bugzilla.gnome.org/show_bug.cgi?id=706494