Foam insulation penetrates every crack and crevice of a wall cavity and attic space to create a monolithic barrier which blocks dust and bugs from entering a home.
Dust. In Arizona, dust is a constant presence. Every time there is positive and negative air pressure between the indoors and outdoors, dust enters the home with the flow of air. Because Arizona homes do not have a an air seal, this a near constant occurrence. Foam insulation air seals the home, which blocks all the uncontrolled air flow through outlets, baseboards, ceiling fixtures, can lights, etc.
Bugs. Similar to dust, foam insulation blocks the pathways for bugs to enter a home. This is particularly true for block homes, and older wood framed homes. Homes built up through the 1980s were built very loosely because evaporative cooling was used to cool homes (the looser the construction, the more effective the evap cooler). This loose style of construction also allowed for bugs to freely enter a home.
Foam insulation is a non-organic material so it has no food value for bugs. They will not eat through it to enter a home after the insulation is installed.
Note: Since 1991 the energy code has required an air sealing layer to be installed in all walls and ceilings in homes in Arizona. Unfortunately, this code has never been enforced, and is still not enforced in homes being built today.