yeah, search function kiddies, but as we're all here now...
why are there undercovers at raves? not for you and your couple 'o pills, of course not. they are looking for someone dealing. but then why dont you see anyone getting busted? well they also arent interested in the guy selling 10 packs.
so they follow him, and take down his license plate number. it goes into the database. if he is particularly active they might follow his car, especially if it looks like he's off halfway thru the night to pick up more supplies.
they follow him, and find the house of his dealer. more info for the database. follow a few more people and suddenly you have a clear picture of whats going on.
(sorry for all the male pronouns)
this all make sense so far? lots of information, no arrests. that's how it works. maybe they will use that information later to arrest someone, but thats not the point of the operation on the night.