Why is the location of a building in relation to the property line important from the standpoint of stopping fire spread?

A fire on or near the property line may have enough power to ignite adjacent fuel, including other buildings hundreds of feet away. Fire-brands (embers) may be carried by the fire's convection, supplemented by the wind. Radiation from a large fire may be powerful enough to ignite fires inside nearby buildings, where the radiation enters through the Windows.

Some urban fire codes prohibit opposite-facing Windows in adjacent storage buildings. In other Words, Windows must be offset to prevent the radiation from fire in one building from being projected into the next building through its Windows.