They have built-in timers so you brush the proper amount of time.
Did you know you're supposed to brush your teeth at least 2 minutes at a time, spending at least 30 seconds in each of your mouth's four quadrants (upper right and left sides and lower right and left sides)? You probably think you easily brush your teeth for that amount of time, but if you timed yourself, you might be quite surprised at how little time you actually do brush. The average brushing time for Americans is a measly 31 to 65 seconds per session, depending on sex and age [source: Radius Toothbrush].
One of the more helpful attributes of electric toothbrushes is that most come with timers that beep when two minutes are up. If you haven't heard the beep, keep brushing! Others additionally emit a beep after 30 seconds, so you know it's time to switch to another section of your mouth [source: Electric Toothbrushes].