You don't have to understand it, but you should be able to extract data from it.
It used to be there because PropertyValue started out as the fundamental variant type before shifting focus to being the fundamental boxed type.
If all you're doing is setting properties, you can do it any way you like.
cheap_steady_clock faster than std::chrono::high_resolution_clock ?
It's quite a bit faster, though maybe you don't care.
If the callback requires copyability.
It's more important to clean up when you leave.
Remove it from the equation.
The fact that a property has been set does not by itself trigger functionality.
The default value must be legal.
Setting it to the value it already has is not a crime.