modify helps support in-place mutation, which can help ARC optimizations. Consider the following code:
foo.x.append(0) // `foo` is an instance of `Foo` from Joe Groff's example
If Foo.x were to use get/set, then x would return a new Array instance referencing the contents of _x. append(0) would then mutate that array, triggering a copy-on-write, then that array would be written back to _x.
Since Foo.x uses modify, x will yield _x's Array instance so that append(0) can directly mutate _x. If _x's backing storage is known to be uniquely referenced and there is capacity for a new element, then this will avoid a copy-on-write.
IIRC this is why modify is already used for Array's subscript (using _modify since it's not yet a stable feature).
You may also want to read this proposal for more information about modify and yield.