Most fires start in the contents of a building.
For example, a smoldering cigarette starts a fire in a stuffed chair or mattress. It spreads throughout concealed spaces, poke through walls, common roof or attic spaces. Sometimes even along the outside of the building. Extinguishing a structure fire, is much more complex than quenching a content fire. The concealed flames must be located and cut off, in addition to extinguishing the original content fire.