When a house is famed there are places where air would be able to flow from one paart of the house to another inside of the walls, floors, and chases. When air can flow, any fire that ever starts can also flow, and a chimney effect can and will accelerate the flame spread and temperature of the fire, hastening destruction of the building and placing lives in danger, both that of residents and firefighters.
To prevent this, codes and common sense call for blocking to be placed such that air flow is reduced or eliminated. Theoreticly eliminated, but some cracks will allow a bit of air to move, but the hazard is slowed to perjhaps an hour so there is time to get out.
Fireblocking requirements are stricter in multi-family buildings than in single family units.