The ones I've seen to find something were sniffing for contraband imports to USA. Of course, elsewhere I've seen some sniffing for narcotics and explosives, but the only "live action" I've seen was when they did confiscate an apple from a passenger who just arrived in SFO. Not an Apple product, but an apple.

Agro produce regulations, probably. I had some produce confiscated once too. This is designed against various pests that could enter from overseas and from which local species have no protection, though I think it is futile because if they would get here they'd get here on cargo ship with thousand tons of apples, not in somebody's one apple.

Agreed. But I'd say the agro produce regulations, and sniffing the hand baggage of air passengers, are a bit similar thing for food hygiene as TSA checks are for air safety: an illusion that something is done.