Direct vehicle-to-vehicle communication can be used to provide a lot more detailed localised info (e.g. heading, speed, current position) on a lot shorter timescales (+ end to end data delivery timelines guarantees) for collision avoidance and warning systems. The current intended standard is to broadcast these basic safety messages at 10Hz but that could be higher with 802.11bd and NR-V2X. Additionally there is dedicated spectrum set aside for vehicular communications on an international level, which helps ensure there is less congestion for these safety critical applications.

Using localised peer to peer comms also eliminates some of the privacy concerns with directly sharing this info with a centralised authority as well as removing the need for any global persistent identification for this info. They also eliminate any reliance on the cellular network/internet which cannot be guaranteed to function normally for safety critical communications.

There are also alternate cellular standards for vehicle to vehicle comms called C-V2X and NR-V2X so it's entirely feasible one cellular radio could do general data comms and the P2P safety standards.