Why in CSI do they never turn a light on in a room?

They're always using their flashlights when they go into a dark room even though there's quite obviously electricity in the house! What's wrong with the light switch?!
Update: They always wear gloves and therefore can't leave their prints on it. Also I don't think it's because they want to see how the killer saw it because they can't control the sun if the murder is outside!
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