What is better for a roof, plywood or OSB?
It could be that they are trying to slow down a fire from spreading. The material at the edges might be more fire resistant, so the fire will spread to adjacent buildings more slowly, allowing the occupants more time to escape.  You might be able to verify this by checking the local fire codes for conjoined (not sure this is the right term) buildings.  

See how if there was an MS Paint fire, the flames would stay away from the next building (assuming the material in the middle burns faster).


The man would then have more time to get to safety, or be saved by Captain Construction.