When two persons test bedding together, your partner's position on the mattress should not be noticed and vice-versa. The box spring or foundation supports the mattress. It is generally much firmer than the mattress unit, so that the comfort can be designed into the mattress. You want the box spring or foundation to provide support. If you want that floating feeling, buy a waterbed.
Buyers perceive the box spring to be better and generally it is.
That is because foundations usually appear in lower end products and have minimal insulation. A foundation that is properly insulated with polyurethane foam is as good as a box spring and costs just as much. The function of the box spring is to support the mattress and facilitate its desired comfort. This function can be achieved with steel, wood, or a combination. Manufacturers can jump the coil count by using a mattress unit as a box spring.