two reasons one would want to intentionally turn off traction control:
1. if you are stuck in mud or snow. As much as it seems counter intuitive to turn off traction control when you need traction, the system has a hard time understanding you are stuck, therefor it will cut the power of one or both wheels to prevent slipping. this doesn't help you get out of deep snow and often will bury you further into the hole your car is digging. By turning the traction control off it allows you to generate full power to the wheels to hopefully catch and get you out of the rut you are in. I am sure there are videos on you tube of how to do this properly. I do it in my RWD car to get unstuck.
2. Performance. lets say you are out on a sunny dry day, empty back road and want to really experience the full power and handling of your car. Turn your traction control off to allow the wheels to receive unrestricted power when cornering or accelerating. Normally traction control would kick in and cut the power to your wheels to prevent you from squealing your tires or sliding in a corner, but if you are experienced you may want to push you vehicle to the limit. Make sure you are confident in your driving ability and know your vehicle inside out.
Those are the only two reasons to turn traction control off, if you are really stuck or want to push your car to its limits and feel all its power. Otherwise keep it on and drive safe :)