If a function takes an array of Animal as an argument, the "intuitive" assumption is that you can pass in a Cat[] to it since it's probably processing the array items and a Cat is an Animal.

In usual Javascript patterns, this intuition works, but it's not sound, since arrays are mutable; the function could write to the array, appending a Dog or a Potoo or a Jellyfish- valid for an Animal[] argument, but not sound for a Cat[].

In practice, in most JS code I've seen, arrays are constructed once and then are passed around as immutable collections, so Typescript's unsoundness saves a lot more casting than it introduces errors.