This has potential
It can get a bit annoying/distracting if people are having a conversation when you're trying to race. With a private chat function, spectators would still be able to talk without drivers having to block messages, and the people still driving would still be able to communicate with each other during the race.
Or, and this is pretty revolutionary, the guys wanting to have a conversation could leave the server and talk somewhere else, and leave the racers in peace!