The bigger picture would be: 1) Don't give a reference to your mutable private/shared variables because it's dangerous. 2) Because we still need to let people

The bigger picture would be: 1) Don't give a reference to your mutable private/shared variables because it's dangerous. 2) Because we still need to let people read this data, give them copies (of a potentially large structure) instead and lose performance. It just happens to be that arrays are a common case.