Why do you fill cavity walls with insulation foam?

Insulating foam is just one thing that can be used. Insulating the wall cavity stops heat exchange with the exterior which is a problem during extreme climate conditions like winter and summer. Insulation works by stopping air movement through the cavity. The real trick of insulation is that it isn't the insulation material itself that stops the heat exchange, it is the air trapped in it that does. Air is a poor conducter of heat energy provided that the air is stagnant (not moving around). When applied, the foam bubbles up as it expands, trapping air in the bubbles as it hardens making it an Excellent heat insulator by virtue of the air trapped in it.