There are basically two types of handpieces, air-driven and electric. Electric handpieces are much stronger than air-driven due to the fact that a motor is spinning the bur as opposed to air.
This allows the doctor to remove decay at a faster pace. This will cut down on the time it takes to complete a procedure and lower the risk of nerve/pulpal damage during crown/decay preparation.
Slow cutting tends to burn the tooth, causing nerve damage, leading to the need for root canal treatment, and in some instances, even loss of the tooth.