Array when mutable acts like a list: items are indexed by an integer, and you can replace, add, and remove items.
Array, when immutable acts like an area of contiguous memory: items are indexed by an integer, and you can replace items.
Dictionary, when mutable, acts like a map: items are indeed by key, and you can replace, add, and remove items.
Dictionary, when immutable, acts like a read-only map: items are indexed by key. You cannot add, remove or even replace items.
That is inconsistent. I think they would be better of adding List as a third container type, and changing Array to a fixed size (at creation time) container. Immutable Lists and immutable Arrays would behave exactly the same, but I don't see that as a problem.
On top of that, they chose to make Arrays and Lists value types, (prematurely?) optimized Arrays (but not Dictionaries) to copy-on-write their data, and decided to make that manual copy-on-write (to ensure that an Array is yours and only yours, you have to call unshare on it)
My disclaimer is that I only read the ePub that Apple provided, and somewhat partly so.
I imagine the point is to have a low level type describing an area of contiguous mutable memory, rather than expecting people to use arrays everywhere. Not that I know anything about Swift of course.