Why do you inject v brine (or vice versa)?

Perry Lang has a discussion of this topic in the book titled "serious BBQ."

Brining and injecting (or even marinating) aren't necessarily mutually exclusive approaches to preparing meat.

Brining, at least from my understanding, is using a salt based solution to change the protein structure of the meat, allowing it to preserve moisture while cooking.

Injecting doesn't do this. It injects herbs, spices, flavors, or liquids into the meat directly, avoiding the brining and/or marinating process altogether, which results in a completely different variable into the cooking equation. Not only does injected meat not have the "brining barrier", it isn't "pre-tenderized" as proper marinades are oft to do. I say "pre-tenderized" in quotes, because marinades don't really tenderize meat at all, they pull a trick play which makes it seem like the meat is tenderized.

Marinades are acid based solutions, which "break down" the surface of the meat being marinated, allowing flavors from the marinade to substitute the liquid lost during the marination process.

This next part is just theory, and since I have never tried and don't have any education on this particular matter, it may just be food for thought. hah!

I think a combination of some or all of these techniques may make for interesting results.

Brining forces the meat to retain juiciness while cooking.

Marinades break down the outer walls of meats, allowing flavors to penetrate the meat whilst substituting water lost during the marination process, and also yield a seemingly more tender texture.

Injection artificially increases the amount of liquid and flavor inside a cut of meat.

So who's up for brining something, marinating it, and also injecting it? Hah, probably overkill. But fun stuff to think about