It streamlines the data path by eliminating processes that could add latency to the image. All fixed pixel display have some built in latency-- latency is the time between when data is received by the display and when it is shown by the display. In some sets this latency (or lag) can be significant. In most games your timely reaction to events on screen determine your success. If too much lag is introduced your reactions will be detrimentally slow due to the fact that the events on screen have already happened and you're reacting to them after-the-fact. This is particularly crippling to fast action games such as first person shooters and especially damning to rhythm games like rock band.
Plasma, in general, has enjoyed recognition for having less latency than LCD (which typically introduces noticeable lag even when placed in it's comparable game setting). Still, game mode on your ut50 ensures you are getting the fastest reaction times available on your display. Be sure not to introduce any lag inducing image processes like frame interpolation (smoothing) or image resizing (under scan).