Why are foams put inside the exterior walls of a house?

Foam is inserted into walls for two main reasons. Insulation is the primary reason, while fireblock is the second.

Foam insulation can be injected into the wall of an existing building without much destruction (typically just a series of one-inch holes to be filled). It increases the R value of a wall from about 2 up to about R11. It also cuts off air leaks and can cut off entry points for pests.

Some (not all) foams are highly fire resistant. They can keep fire from racing up through the stories of a home.

Foams also can help with sound deadening.