Fire safety is a major motivation throughout the building code, and this rule is kind of a proxy to fire risk assessment. If you join rooflines then fire can ju

Fire safety is a major motivation throughout the building code, and this rule is kind of a proxy to fire risk assessment. If you join rooflines then fire can jump from one to the other. Larger structures and structures used for residence are high risks for fatalities if a fire spreads too quickly and/or without adequate ability to evacuate. Structures assembled ad-hoc over time tend to be high fire risks for several reasons, and avoiding permitting and inspection requirements avoids any check on that.