>>>What on earth do you do with mice? Not feed them, I hope to other animals. :( <<<
Actually, yep, it's usually for feeding to other animals. Most anyone that owns reptiles has to feed them live (or recently alive) small critters. And many other 'pet owners' whose pets naturally eat protein, use them in their food.
Compared to the confinement lots where animals are poisoned & tortured before they're slaughtered to provide 'food' for humans, I find breeding mice for pet food to be more in line with nature (with the caveat that some might not feel that having a reptile in a cage for a pet is 'natural').
The circle of life (even in nature) is sometimes very brutal, eh?