The soft base is more used with the lower complete dentures. It works like a cushion that helps absorb part of the pressure exerted on the gingiva during chewing. Since it is porous, the soft base adheres better to the gingiva, so it improves the stability of the lower complete prosthesis in the mouth and decreases the discomforts on the gingiva.
However, dentures can remain unstable with a soft base. In order to permanently remedy this problem, dental implant placement would be the best solution.