Around the perimeter, both vertical and horizontal insulation (pink) provides a thermal break. During the winter, the insulation prevents cold exterior air from entering the building through the ground. The vapor barrier (yellow) protects the slab from ground moisture.
The rebar provides structural integrity to the concrete, tying it together and adding strength as external forces (tensile strength) pull on it in various directions over time. This will reduce the potential for the concrete to crack in the future.