Sport mode is most useful when you want to explore your automobile's full performance capabilities. Since this setting will likely make the throttle response sharper, the steering heavier, and the suspension not quite as comfortable, you'll want to avoid using it while commuting.
What is the point of sport mode?
At its simplest, engaging SPORT mode makes the vehicle's throttle more sensitive, for hair-trigger response. The automatic transmission may respond similarly: downshifting earlier and holding higher revs for longer periods to keep the engine's power output within striking distance.
Should I use sport mode on highway?
You can definitely switch from D to S while driving, just don't do it while pedal to the floor. Even that is probably safe as the computers won't let it hurt the car, so it will only shift when its safe to do so regardless of what you ask it to do via the lever.