> do one thing when field a is null and field b is not

And that would have been a good example! contrary to the one in the docs.

Pattern matching for checking fields of things of the same type is a good use. Pattern matching when checking your parameters type? bad.

> The nice thing about it is the compiler can assert that your match is exhaustive

If you pass a new Shape object for which you forgot to implement a new case in the pattern matcher, you just get the default case, which is wrong, but the compiler can't know that. Now have the function argument ask for something that implements IArea interface and if you don't implement getArea() in your new Shape class, the compiler will know.