We’ve used it for a couple of things in the past. It’s useful for tuning PIDF by only putting PID tuning things onto network tables when in test mode. It’s useful for auto-setting max speeds of the drive train and mechanisms just from being in a specific mode, which allows demos to be done safely no matter where we are.
Last year we had a programming workflow where new hardware modules were enabled in testPeriodic first. This allowed us to test new modules when needed, but then swap back to known driver modes by going back to teleopPeriodic(). This worked ‘ok’, but wasn’t the best.