Along with preventing air leaks, caulking around windows and trim provides protection from outside elements seeping inside, around the window frame or through other gaps. Dust and dirt enter the structure more freely without caulk to provide any blockage. When the moisture from rain or snow enters, not only does it damage the structure--causing rotted, warped wood and peeling wallpaper--it also leads to dangerous mold and mildew. Water and mildew stains appear on the walls, and also in the carpet.
Caulking provides a seal against small insects that would otherwise have open routes to enter a house or building. Also, without caulking, insects have access to more hiding places once they enter a structure. In addition to creating a more professional-looking appearance around surfaces and materials, caulking also prolongs the lives of those surfaces. Because it protects against outside elements and thwarts inside water damage, caulking also helps protect against erosion.