Why do you soak them in salt water?!
Three things:
Aging them is to get past rigor mortis. I think you understand that.
Brining pretty much means salt and a wet solution, usually soaking in a liquid. The salt keeps moisture in, either water, natural juices, or stuff in the water. This is helpful if you cook them with a dry method like grilling, frying, or roasting. If you cook them with a wet method I don't think it does much, if any, good. You can add salt flavor any time you wish in the cooking process.
Marinades get confused with brining sometimes because some people do both together. A marinade is acid based, wine, vinegar, or tomato products being pretty popular but there can be others. The acid breaks down fiber which tenderizes the meat. This is helpful with older birds, not all that useful for young birds like Cornish X butchered very young. Marinades can also be used to add flavor. They don't absolutely have to be acid based, it could be just for flavor, but usually they are.