what purpose does the cavity in cavity walls serve?

The cavity creates a void between the inner and outer “skins” of the brickwork and blockwork. The cavity allows for insulation of a home therefore increasing the heat that will be retained. As well as to stop water from reaching the inner skin (blockwork) which will create damp spots, as the outer skin (brickwork) is open to the weather at all times the cavity will prevent any water from travelling to the Blockwork, there can be cases of water reaching the inside blockwork if the cavity has been bridged which means there is something that is allowing water to travel along it two both skins. Thanks

The cavity purpose is to allow air flow around the cavity to prevent any damp ,also allowing any water ingress from the out side such as the roof or any gap water can get through to drop down the cavity allowing no water to penetrate the inner skin