read-only properties are assigned only once - on the initialization of the instance.
It is different from
getters allow the instance itself to change the value of the property. It is different from constants, since constants are assigned on the class declaration.
One use-case for read-only properties is to define immutable objects, another is in the definition of
static read-only for a singleton implementation.
Other use-cases may be in container properties (like lists and dictionaries), in which it is intended that their contents might change, but the collection itself stays with the instance for all its life-cycle. This way you can be sure that they are never nullified.
These are some use-cases of the top of my head, I'm sure that there are more...