It was already said before, but I would like to emphasize, that each motor has a limited thermal resource.
Let say you have two similar robots of nearly identical mass, with one geared directly from 600 rpm cartridge and another geared down from 600 to 400 rpm.
Then, if you want to run them at the same speed, you command direct drive to 400 rpm, and one geared down to 600 rpm at the motor => 400 rpm at the wheels.
Since gearing down speed also gears up torque, it should be obvious that motors of the directly driven robot will need to produce 1.5 times more torque, than the motors of the geared down robot, to achieve the same acceleration profile.
more torque => more current => more heat
Anyone taking high school physics will learn that torque that motor generates is directly proportional to the current flowing through the motor. And the amount of heat generated in motor windings is proportional to the current squared times the resistance of the windings.
The more torque the motor has to produce, the more internal heat it will be generating and the sooner firmware overheat protection will be triggered, limiting max current and torque of the motor.
If robot is light enough not to trigger firmware overheat protection, then you may be fine running direct drive at lower rpm. You will even save more weight by not having extra gears.
However, if you want to carry more weight, without the risk of overheating and stalling, then you will be better off gearing it down to reduce torque demand on the motors. There is no way those gears will add an extra 50% to the robot’s weight.