Redundancy. Each rover echos it's findings -- images, raw instrument data, weather data, etc. -- to at least 2 other rovers, chosen randomly from the rovers that are online. Every time an instrument or camera is ready to send a load of data out, it sends it back to Earth and to another rover, so if a rover goes offline or if something prevents data transmission (weather, solar, mechanical failure), the data isn't lost and the mission can continue despite the failure.
Pings. Each rover pings the other rovers as a "heartbeat." If one of the rovers doesn't respond to the ping, then there's a problem, and the rovers report this back to NASA.
Duplication of effort. The rovers send each geolocation data. If two rovers happen to accidentally be on converging paths, they warn each other off and stop until NASA can assign new paths (or their internal AI can assign new paths).
Warnings. If rover 1 detects an oncoming dust storm, it sends out warning data to other rovers with wind speed and direction. Downwind rovers in the path of the storm can then plan accordingly to go into an automatic shutdown to save energy, prevent system damage, etc. during the storm. By having the warning, they can safely stop in-progress testing before the storm hits.
Distributed computing. If one rover is analyzing data and needs more processing power, it can offload some of the data to another rover that indicates it is (somewhat) idle. This effectively makes the rovers a "cloud computing" system, since some rovers might be at lower system utilization than others. Faster processing means faster delivery of results back to NASA.
Geolocation. By sending timing packets between rovers and between satellites, each rover can get a better fix on their current position and heading. This means fewer satellites are required to provide the Mars equivalent to GPS.
Local network. By having constant communications, there's the Mars equivalent to "wifi" for astronauts on the ground. This means the rovers can be the access points for smaller, lower-powered, robots or machines. This way, every machine on Mars doesn't have to pack the weight of a dish and battery capable of reaching satellites or Earth. They just have to be able to reach the nearest rover, which can then relay data to/from orbit or earth for that device. This allows more machines with smaller mass and lower power requirements.