Brew Your Own magazine provides a great article on making your own stir plate. Having a stir plate allow you to keep your yeast starter in constant motion, thereby allowing the yeast constant access to it’s food source and oxygen. Your starter will end up being much more active and healthy which is good because it lowers the risk of bacterial infection during primary fermentation.
Another benefit of the stir plate is that you get to feel like a fancy scientist.
The stir plate is super cheap to make and possibly free if you have the parts laying around. If you don’t, mostly everything you need is available at your local hardware or electronics store.