These are called Property Observers:
Property observers observe and respond to changes in a property’s value. Property observers are called every time a property’s value is set, even if the new value is the same as the property’s current value.
Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/ca/jEUH0.l
I suspect it's to allow for things we would traditionally do with KVO such as data binding with UI elements, or triggering side effects of changing a property, triggering a sync process, background processing, etc, etc.