It's all about flexibility. Below are a few things I've had to do and it was much easier because each building had it's own AP group.
1) Reporting in AirWave by building (ap group)
2) Enable a temporary SSID in a single building
3) Disable ARM OTA for a specific building where students reported issue with their playstations.
4) Enable 11b rates in a certain building because a certain device was in use.
5) Have some buildings point local 1 and some point to local 2
6) ClearPass rules based on building (ap group). For example: one building may have a different captive portal from the rest.
7) Different channel width (20, 40, 80) by building based on AP type, use type and environment.
Those are all the ones I can think of right now.