How is sous-vide different from simply cooking the chicken?
It's actually not that much different in some respects.  Sous vide is simply a way to precisely control temperature (that's something you can't do very easily with a poach on the stove top).  If you sous vide a chicken breast, remove it from the bag, and serve it up, it will be very much like the result you get from poaching. The difference is that you will be able to control the level of doneness, and impact the texture to a greater degree than simple poaching.  Most sous vide ends with a searing or browning step, so that the exterior of the product more closely resembles a stove top or oven cook.