The granular material can be 'flattened' better than the sub-base and results in a more uniform thickness for the slab.
Consolidating the fill also consolidates the sub-base and results in a more uniform base and sub-base stiffness.
Both help improve the quality of the slab.
Both the ACI and Portland Cement Association published material several years ago that indicated that the increase in the load carrying capacity of slab is only marginally improved by the addition of a granular base.
The granular subbase is also used for drainage. One of the most significant problems experienced with rigid pavements is that of "pumping" at the joints. This results in loss of material at the joints and a corresponding loss of support for the slab, resulting in cracks and faulting.
Granular subbase prevents the buildup of pore pressure immediately beneath the slab and allows capillary drainage as well.
The large size aggregate creates large size void spaces. These large spaces provide a "capillary break", thus preventing water from being sucked up to the underside of the slab by capillary action. Capillary action is the phenomenon of water travelling through tiny spaces without the need for any pressure to push it along.
So if the slab was placed directly on (compacted) soil, ground water would rise up to the top of the soil, through capillary action, and saturate the bottom of the slab and eventually migrate through the slab to the surface.
Thus the 4 to 6 " layer of 3/4" stone helps prevent water (in liquid state) from reaching the bottom of the slab.
Water in vapor state will still come up and either be stopped by a "vapor retarder" (plastic sheet) or if there is no vapor retarder, travel through the slab.
It will then evaporate into the air, or be stopped by a floor covering and condense (not good).
