These give added protection to the carpet against the hard subfloor during normal walking.
Indentations from furniture will be less severe where the carpet is fitted on a firm underlay.
Firmer underlays will prevent excessive vertical movement and stretching of the carpet in use and will therefore help to prevent rucking problems.
Soft, low density underlays do not give adequate support in heavy wear areas as the carpet pile is effectively squashed against the subfloor, accelerating wear and appearance changes.
A softer product will collapse under the pressure and allow the carpet backing to be distorted, preventing good recovery and making indentations more severe.
Thicker, softer products have been seen to create dimensional stability problems in some installations where the carpet has stretched due to the excessive vertical movement as the underlay is compressed during normal use. This can cause particular problems on seams.
In our experience it is false economy to fit new carpet over old or softer, cheaper underlay. As already mentioned, carpet needs to have as firm a support as possible in order to achieve its maximum performance. An old flattened or new soft underlay will not be capable of providing adequate support to the carpet.