Why would half my attic roof be drywalled
It's an "approved" substitute for a "fire wall" that does not extend 30" up through the roof.  

The Code requires fire walls (and sound walls) between multi-family housing units to extend 30" above each unit. However, one approved "alternate" design allows the fire wall to terminate at the bottom of the roof sheathing, if gypsum board extends out from the party wall 5' (I think) on the bottom of the roof joists. (The portions that are missing, could be sloppy work that has already fallen down and removed.)