Why are battery-powered appliances and tools specified in volts?
For motors Power is proportional to torque times the rotational speed.  So for a given rotational speed and torque the device produces a given amount of power.

To increase the amount of power two options exist.  Generate the same amount of torque at a higher speed, or increase torque at a given speed.

For a cordless drill, the speed is normally variable and depends on the application.  For instance high speed for steel, lower speed for masonry, and lower speed again for wide hole "auger" bits in timber.

Ok so to increase the power of a cordless drill you will not change the speed, as the drill needs to deliver power at a variety of speeds.

Two other factors to consider, in a DC motor, voltage is proportional to speed and current proportional to torque.

But all the designers are doing is increases the pack voltage.  for a given coil resistance in the DC motor, increasing the voltage across the coil also increases the current, thus the torque delivered.

So increasing the voltage is a way that designers can increase the torque, thus power the end users can use!.  So more volts the better!  upto a point as more volts means more cells, and more cells means more weight, more weight means more user fatigue.  So these tend to balance out, at the moment anywhere from 14.4 V DC to 18 V DC for a typical cordless drill.