Structural Foam for chassis stiffening
Most of the foam in new cars is for energy absorption (occupant protection in a collision) and sound control.
Different densities of foam are used. For occupant protection it's ridged foam. For sound and panel flutter it's often a flexible foam.
The plastic gussets mentioned above, snap into the inner structure and work as a barrier to control the foam when it is applied. They prevent the foam from going where you don't want it it go.