They are a great place for/to:
Involved in gathering several Badge of Vows in the DLC noticeboarb quests of the "From a different sky" series.
Rooftops are a great place to look for chests/loot. In Gran Soren you need to use the rooftops to access balconies to retrieve certain Notice Board Quests items like the War Bugle. The DLC Quests "From A Different Sky" Parts 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 require you to find several Badge of Vows on the rooftops.