It's basic chemistry..the secret to a moist bird is not to dry it out, obviously..we've all had turkey that is dry..and near inedible..OK..here's my recall from basic HS chemistry...when you have the bird soaking in a saturated solution of salt and sugar, the water inside the bird is less salty, right, so water flows INTO the bird as the solution tries to achieve balance....you cn actually measure it..if oyu have an accurate scale..wieh the bird before you put it int he brine, and then after you take it out and pat it dry...the gain in weight is the extra water the bird has absorbed..which makes it moister when it cooks..
Let me make a suggestion to you...to convince yourself, and not to worry about ruining your tThanksgiving..do a test this weekend...buy a package of two whole chicken breasts..brine one for 24 hours..just make a smaller amount of brine...and then roast them both at the same time..though use separate pans..for the same amount of time...I guarantee you that you will be able to noticably taste the difference..