Packing peanuts are chicks (usually male) that a hatchery includes in small orders so that the chicks can keep each other warm during transport. They are free and if you do not want them you have to explicitly tell the hatchery this. I got no packing peanuts because I do not want roos and I called meyer and informed them of that fact.
Sometimes there can be quite a few and if you are not prepared for that you may run into trouble.
It could be 2 things.
The standard packing peanut = styrofoam shaped like a peanut to fill empty space in a shipping box.
The one you're referring to, I think, is a surprise chick. Tends to be a left behind chick, more than likely not the same breed you ordered. They're considered "extra" or "unwanted."
As for the "poor" roosters, well.....you just gotta face it. Nature produces 50-50 males and female. Yet Billions, yes B, billions of hens are wanted and needed, while roosters? Not much use for them. I do not know how high the percentage is over hatched roosters who "make it" to live, but I'd guess, world wide, less than 2%. The rest are destroyed.
They can also keep your chicks warmer in the case of a small order, less than 25 chicks. In today's world of small chick orders for backyard farms, it can be a good thing. You can sell or give them away on Craig's List or through BYC. I'm raising a bunch ( 15) for dog food and chicken soup/ stock. I have no hopes of being able to let them getting full sized since they are mostly Roos and I'm in a city neighborhood, but even if they weigh 2-3 pounds when processed its a bunch of nice stock and fresh meat for the dogs. My chicks that I ordered arrived safely and I helped another BYC member at the same time by taking her packing peanuts. So, I'll just raise them until they start crowing obnoxiously or causing other trouble. Right now they're fine, some are 9 weeks, some are 7 weeks.