Mobile Phones are devices used for communication purposes.
In Watch Dogs, everything is connected through the mobile phones, which contain every information of the owner, which includes their name, status, job, location etc. The mobile phones in the game are directly connected to the Central Operating System, or the CtOS.
The player can check out if a NPC(Non Player Character) is holding a mobile phone. This can be detected from the mobile phone icons over their heads. The mobile phone is also required for most of the Profiler abilities.