The shape of concave mirrors catches and directs the sun light inward which provides greater heat retention. This allows the heating system to achieve a higher temperature and be better suited to heat your entire home. A flat or convex mirror would reflect a lot of the light outward, thus making those shapes less efficient for gathering solar heat.
Concave mirrors absorb all the incident light and reflect it to a single focal point. This reflected light is very powerful because it carries a lot of thermal energy. The cooking pot is placed on the focal point where the light is concentrated. The energy is powerful enough to smelt weak metals and melt salt.