Originally Posted by zkzkz

You can be sent to secondary for either immigration or customs purposes. That is, they might have already decided you're admissable but want to check that you're not smuggling anything.

They're not always very clear why you're at secondary though. I've had an agent clearly tell me she didn't believe I was traveling for the stated purpose and send me to secondary. After sitting around for two hours they said they were only interested in searching my bag for customs and didn't need to speak to me about immigration issues at all. They didn't even look very hard at my bag either. I think they couldn't figure out why the first agent had sent me there at all.