They're to let everyone know when the food in front of them should be thrown out, there's a 10 minute 'holding period' in which the food is still okay to be served after it has been cooked and placed in the bin.
Generally it's the kitchen manager who keeps an eye on time cards, as it's their side that is right (red or black number) and matches the clock, and they will tell someone on front to throw the particular food away.
For chicken it's a 20 minute holding period (actually this may vary depending on the chicken product, I'm not quite sure as I've never worked kitchen) but as it's made as its ordered these don't have time cards but have their own timers on the UHC.
I don't see why you'd get shouted at for dropping them, it's not like they touch the food, as long as they get picked up and get put down the side of the production bin I don't see the problem.
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