Probably one of two reasons. 1. Control. They would control the medium, an thus have full control over the communication. Maybe even apply wiretapping if neede

Probably one of two reasons. 1. Control. They would control the medium, an thus have full control over the communication. Maybe even apply wiretapping if needed. 2. Anonymity. P2P usually means that you know the address of the other participants, with a central server you don't get to know their address. Professionals might need this extra layer of protection.