Why do some people clean their plates to the point of mopping up even the crumbs with a piece of bread?

I always have something left on my plate. I can’t say I’ve ever completely cleaned my plate. But I’ve eaten with a few people who just seem almost frantic when they eat. I mean they eat every thing on their plate. I’m pretty sure that if no one was around they would literally lick their plates clean. The whole thing is very unattractive, and I wonder why they do it.