But if you were riding on a flat course where a quicker start is needed, you would want to use the lower gear. Also, shorter crank arms can allow you to pedal faster. The crank arms connect the pedals to the bike. The longer they are, the bigger the circle that your feet have to turn, and the longer it takes to make a full rotation. The trade off is the longer the crank arms, the easier it is to pedal.
Cadence is another important factor.
Cadence is the revolutions per minute that you pedal. These generally range from 70, which is pretty low, to racers that have a cadence around 110. The important thing for a rider is to find the cadence with which they are comfortable and to then have the right set of gears that work for that cadence and the type of riding they are doing. The beauty of the gears is that you shift as you ride so that you can continually keep the same cadence. Another thing you will notice about different types of bikes is the size of the wheels.