[Mathnerd314]

Say you have an array x = [1,2,3]. You do x[2] = 4. Now you have two arrays, x_1 = [1,2,3] and x_2 = [1,2,4]. x_2 is a "copy" with all elements the same except one. The "always copy" here means that (semantically at least) every array modification creates a copy of the array like this. Naturally this is slow, if you implemented it as-is, but then there are the optimizations I mentioned. And it is much easier to optimize a simple semantics consistently than to debug a more complex one. It is somewhat similar to Swift's value semantics.