It's mainly so moisture doesn't get close to the underside of the floor, and also to allow me to fit the clamps (the orange things) to the joists so I could tighten up the T&G
My reflective foil isn't perforated so I've cut a slice in every droop to allow water to escape.
The foil should reduce rising moisture and will also reflect a tiny bit of temperature and reduce updrafts when the T&G opens up, but it doesn't have significant thermal properties. It was cheap as all hell so I fitted it anyway.