Google Maps can get you traffic information. Not so hard in principle to add other things that might cause a user to avoid somewhere or choose a different route. My guess would be, this app might have included features specifically for rioters themselves, as opposed to just for innocent folks looking to avoid trouble. In other words, it packed a riot-organising tool.
Given that governments around the world claim the authority to turn off/block social media at times of crisis (and India has gone much further than that in Kashmir without even the provocation of HK riots), it says something about the liberal tolerance of HK that they still have access to 'normal' social media.