Firmer underlay gives added protection to the carpet against the hard subfloor during normal walking. It also reduces indentations from heavy furniture and helps prevent creasing by limiting vertical movement and stretching of the carpet.
Soft, low density underlay doesn’t provide adequate support in heavy wear or high traffic areas, as the carpet pile is effectively squashed against the subfloor, accelerating wear and reducing appearance.
An underlay that is too soft will collapse under the pressure, distorting the carpet backing and preventing good recovery. Any indentations will be more severe and movement increased, especially where heavy furniture is present and moved around the room occasionally.