They keep you from brushing too hard.
It's ironic -- you try so hard to get your teeth nice and clean that you wind up brushing too hard. Most often, this means you injure your gums, possibly even causing some gum recession. (And gum tissue never grows back.) Brushing too vigorously can also remove enamel from the tooth surface, causing sensitivity to cold, heat and other stimuli [source: Mama's Health].
One of the major benefits of electric toothbrushes is that it's nearly impossible to brush too hard with them because you shouldn't really be doing the brushing. With an electric toothbrush, you simply hold the brush and let its moving bristles do the work. You do reposition the brush over different parts of your mouth, but you're not supposed to be vigorously moving the brush back and forth, and you definitely shouldn't be applying pressure.
Some models even have sensors that will automatically reduce the power if you start brushing too hard [source: Electric Toothbrushes]. This is a great option for those who are prone to using a little too much force when they brush.