Cutting tools and drill bits work best when run at the correct speed for the material being cut. Too fast or too slow can result in inefficient cutting, rough surfaces, heat buildup, premature dulling and/or tool breakage. References such as Machinery's Handbook have extensive tables of "Feeds and Speeds" for various types of cutting tools and materials.
But how do you know how fast your machine is running? Some machines have a variable speed control, but no precise feedback on the actual speed. The Pro Trinket Tachometer attaches to your machine to sense and display the actual rotation rate.