It provides warmth. This is good since turtles are cold-blooded animals and need some way to heat up. It controls shell fungus. You turtle needs to be warm and wet, but you can control the darkness that fungii likes and help you turtle too by investing in good lights. There is more to the lighting than just making the habitat bright.
Turtles require special types of light to make sure they are healthy.
UV-B is provided by special flourescent reptile bulbs. A good idea is to have one of each of these bulbs for your tank. You may want to invest in a timer to simplify maintence and control algae. More than 12 hours a day of lighting is asking for trouble.