I've done it in the floor over an unheated integral garage, because I was laying a new floor so there was little extra effort. It does cut heat loss.
Otherwise, heat loss from floors is mostly draughts, especially round the edges of the room and under the skirting. An old house may have draughts through the gaps round joist ends built into the brickwork, or the roof of a bay window or porch. There is no convection, little conduction, and negligible radiation loss through a floor. Seal any cracks and gaps, e.g. round pipes, with fireproof expanding foam. In a terraced house, odours from the neighbours can get through, expecially if they are smokers.
Bare floorboards are colder and noisier than carpets.
It slightly muffles noise, but if that is your aim, use denser mineral wool batts, as the weight improves noise absorption. Cavity wall batts are denser than loft rolls or you can buy a soundproofing grade at higher cost. I now do it whenever replacing a floor.
