Rear brakes allows for a bit more smoothly-modulated power to be applied. Result is shorter stopping distances compared to front-only.
As mentioned, rear brakes are the ONLY ones that should be applied in a tight paceline (while continuing to "softpedal" to absorb the Slinky-effect) on group rides.
Skillfull riders in the middle part of a paceline can even compensate for mistakes made by riders in front of them by using this technique to slightly slow the pace.