The phrase can be used in a formal sense as in commerce, for instance. Imagine the customer walks through the door to make a purchase... That person should most definitely be greeted with the question: "what ya'll need"

Additionally, when out and about with your friends, you notice a stranger looking in your direction, you look back, throw your hands up and loudly say: "what ya'll need"

Or, while driving your car, somebody cuts you off...it would be appropriate to pull up next to the speedy driver, abruptly slow down so as to maintain the exact pace of the other vehicle, roll down the window and as loudly as possible scream: "what ya'll need"