Defaults are usually based on the principle of least surprise: for any given situation, they use whatever is simplest, and least likely to cause problems.

When removing all the items from an array, I think freeing up the space is the right thing to do by default – if you clear an array that was holding 10 million integers, you'd be pretty surprised to see it was still holding on to a fair chunk of memory.

Sometimes, though, we have extra information. For example, if you know the integer array is being cleared but will be refilled almost immediately, you can choose to keep the capacity for performance reasons.

So, Swift does what is simple and safe by default, but leaves open the alternative for times when we know we'll want that capacity again.