A "torque-ier" drivetrain will have greater acceleration. Your robot will reach its maximum speed faster with more torque. however, as Torque and RPM are inversely related, the more torque you have, generally the slower your top speed. That is, unless you add more motors, increasing the resource pool of your drivetrain in a different way.
Think of it like a car transmission.
You wouldn't want to accelerate out of a stoplight while in 4th gear, because 4th gear is designed to give you a high speed, at the expense of low torque. It would take all day to get up to 100MPH starting in 4th.
1st gear has a large ratio, supplying the torque you need to break static friction and get the car rolling from a standstill. However, due to the ratio, 1st gear has a very low top speed and could only carry your vehicle to maybe 15MPH before reaching dangerous engine RPMs. In 1st gear you would never reach 100MPH within the limits of the system. You shift from 1st into 2nd, then 3rd, then finally 4th, to reach your top speed in efficient time*. A
Continuously Variable Transmission (CVT) is most ideal, because you can have an infinite number of ratios to maximize efficiency*.
Torque is also why a semi truck is powered by a 12 cylinder engine instead of a 4 cylinder. While both operate at similar RPMs, a more powerful motor will output more torque. Torque allows the massive, heavy truck to haul a loaded trailer at the same speed as a Prius. Force=mass*acceleration. A truck uses its force (torque=force at along a lever) to accelerate a great mass.
Alternatively, a Veyron uses the torque from its 16 cylinders to accelerate the relatively light vehicle quickly and achieve blisteringly high speeds.
Basically, more torque=more acceleration. Too much output torque, you compromise top speed. Too little output torque, you compromise acceleration. You could direct drive your robot with CIMs, but it would take you months to reach 5000 RPM (if you could move ever at all). You could gear your robot 5000:1, and you would have an ungodly amount of torque, but you could only rotate your wheel once every minute! Of course these are both exaggerations, but there is a careful balance between getting enough top speed, and being able to practically reach that top speed on a 54' field
*When FRC teams use shifting transmissions, they are not using them to improve acceleration, but to allow a selection of high torque or high speed, to deal with match conditions. Depending on the gearbox, acceleration to top speed is only improved by a small fraction of a second when shifting like a car. Their high speed gear may not have the torque to push another robot, so they shift low to assist with dealing with opponents.