I believe the radio rule is in place so that competitors can't listen in on committee chatter about wind shifts and strength. Committee would be discussing those things in an effort to keep the line square and whether marks needed to be moved. The desire is that the sailors would have to find those shifts and puffs for themselves, not by eavesdropping on committee.

There's been an article or two lately about increasing communication between sailors and committee as a way of making regattas better run.