The box spring significantly impacts durability, is designed to absorb/distribute weight & help prolong the life of the mattress. Box springs also raise the mattress to a desirable height.
Box springs have a wood frame & may have steel for strength. Support is added to the frame (a strong wire coil/grid). The sides feature ticking & a skid-resistant fabric to the top.
Posture box is least expensive - a wood frame with a flat top, covered with the same ticking as the mattress. Semi-flex grid has a rigid metal wire grid and minimizes flex to support the mattress.
Dense foam foundation is used on foam mattresses & provides weight distribution to prolong mattress life. Coil Spring provides better support & softness using heavy-gauge wire coils to absorb impact to the mattress.