If you are an overpronator you are at a higher risk for certain injuries and aches and pains than neutral runners will be. This is because your feet do not strike the ground the way they should, which is what allows them to make you more prone to shin splints, bunions, and plantar fasciitis.
If you overpronate your feet receive constant impact which causes these painful symptoms to flare up--and they can get pretty bad, so it is important that you use a shoe that is made to correct your overpronation.