Crime Scene Investigation

As someone who regularly watches CSI, CSI Miami and CSI New York (I'm waiting for CSI Romford to start) I have a question, and I would like some kind of answer other than "for dramatic effect", so in the best tradition of CSI I've set the scene now for the investigation part.
Why is it that even in daylight or in a building where you would expect adequate lighting (such as an office block) the CSI's almost always pull out their torches to look around the scene (and I don't mean the special ones that make body fluids glow either) instead of turning on the lights or opening the curtains?
Mister Squeaky meanwhile has a subsidiary question which he would like an answer for - (although I think the answer is actually quite obvious) why do they all walk around on the ceiling?