It's not a requirement that you have to brine turkeys before they're smoked, but it does improve the quality of the meat. In fact, lots of the frozen turkeys have already been injected with a brine-like liquid that does what brining does, to a smaller degree.
I've brined many of the prebasted turkeys without brining, just seasoning with a dry rub, or injecting with a marinade instead. As long as the breast isn't overcooked, they turn out fine.
Brining helps in a few ways. Because of the way brining changes the meat, it tends to take less time to cook a brined turkey. When done the turkey holds moisture a little better, and the texture of the turkey is a bit better, too.
So brining turkey? Not totally necessary, but does have its benefits.
