1) Say you have a locked door to which you want the key. You put the dust on the doorknob. Then you check the cars in the parking lot for traces. Now you know which people have gone through the locked door and presumably have keys. Checking their backgrounds also gives you a good idea what is happening behind that door.

2) Before shredders got good. You put the luminol dust on the steering wheel of an inteligence officer's car. Then you steel the shredded paper from the base. The luminol then identifies which shreds of paper come from documents handled by the inteligence officer.

3) You dope the inteligence officer's car again. Then you look at the door punchcode panel to see which buttons he pressed. Now you are 90% of the way to knowing the code.