To Use Padding (Cushion that goes under the Carpet) or not to, is a question I am asked a lot when we install Carpeting in a basement or ground level over a slab. We generally recommend using padding unless there is a good likelihood the carpet can get wet, like rooms that are next to boiler/wash rooms, or areas that are prone to water seepage. See below the benefits of padding and of direct glue down installation.
Carpet lasts longer
Direct Glue Down
If Carpet gets wet could be dried using Wet Vac
If you have padding and the carpet gets wet, the padding will have to be changed in most cases.