Brining isn't a must, but once you give it a try it's hard to live without, especially if you notice your chicken ending up a little on the dry side. Brining is not the same as a soaking in marinade, as it's primarily used to enhance the moisture and natural flavor of the bird (while a marinade is to introduce new or bold flavors to the meat).
The physics of it is pretty simple... when you have two separate items of varying sodium content, an attempt at equilibrium is made. Since the meat has a low concentration of sodium and the brine has a high concentration of sodium, there's only one way to acheive an equilibrium... and that's for meat to absorb the sodium. The beauty of it is that when it absorbs the sodium, it also absorbs the water/sugar/other flavors that are in the salty brine. By increasing the water content of the meat, you'll end up with moist meat after the cooking process. By also absorbing the sugar, salt and other flavors of the brine... your intensifying the natural flavor of the meat and adding additional flavors.
I think if you give it a try, you'll notice the difference and be pleased with the results. Personally, I brine all turkey and all white meat chicken (as well as whole birds) that I smoke. If I'm grilling chicken, I'll go the marinade route... but the smoker is more apt to dry the poultry, so I brine.