You don't need to use one. Millions of yeast starters have been made without them. And there is some debate as to whether or not they make much of a difference, but they are cool.
Basically the stirplate creates a vortex and spins whatever liguid is in it, the theory with using them for starters is that they keep the yeasties in suspension and in contact with the starter wort. It also helps to keep plenty of oxygen in the wort to aid in yeast reproduction.
If you are a diyer, the it is fun to make one, or if you have access to one cheap or free it's a good tool. I'm lucky i work in a medschool, and whenever a researcher leaves that stuff tends to get discarded, since new researchers coming in usually either have their own stuff, or a getting grants to get new gear, so I've been lucky enough to have been given two of them, one of them with a heater.