They need to make room for new ones. Make sure if your turtle is shedding skin, keep an eye on them. You might be feeding them too much food, thus causing your turtle to grow too quickly. This loose skin has a habit of clogging up your filter, be prepared to clean your filter at least once a week, maybe even twice, depending on water quality. For more information on this subject try Shedding and Scutes on Valerie's Turtle Information Page.
Turtles are messy eaters.
If you feed them live food be prepared for the occasional body part floating around. They bite and it hurts. Larger turtles can break skin.