The standard library does not currently expose a way to allocate an Array's storage and (unsafely) initialise its elements, as we can with UnsafeMutablePointer.
An example use-case is to implement a concurrent map. See: https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20161024/003769.html
Even though we expose a UMP from Array, it cannot be used to unsafely initialise the Array elements.
The workaround at the moment is to allocate a UMP, initialise its elements unsafely, and then copy the contents in to an Array via the Array(Sequence) constructor, which is less than optimal.