A: Does your set now sag in the middle or is it soft on the edge? Do you feel any soft spots? If you do, it's probable because the box spring is not doing the proper job. The box spring does most of the work and is the most important part of the set. It supports you and the mattress. The mattress conforms to the box spring and is the comfort layer in a total support system. It's the box spring that actually distributes the weight to help keep your body level and must work with the mattress to allow proper spinal alignment.