has to do with stiffness/reliability.
A single piece of wood has the tendency to warp much easier, whereas with multiple pieces of different types of wood (or even the same wood with the grain running in opposite directions) the neck becomes much more stable, because if one piece of wood decides to start warping in a certain spot, the other laminated pieces (whose grains and physical charactistics are different) will lend that piece strength, and keep it from warping.
You may think a single piece would be stronger.. but it isn't. Just think of the woods like people. No one person can be good at everything, they have their own talents, strengths, and weaknesses. Wood is the same. There are certain places within a piece of wood that are weaker or more prone to warpage than others. By "teaming up" with other different pieces of wood, the strength is maximized and weakness minimized.
As far as sound, in theory a single piece of wood would sound better because it would vibrate more freely along with the strings. I dont think it makes a whole lot of difference, but a single very high quality piece of wood will probly sound better than multiple laminated pieces of very high quality wood.