All sorts of bikes get top tube pads not just fixies, and whatever other reasons people have for using them they are merely to protect the top tube from scratches, nicks and dings. Since they get put onto a bicycle, much consideration is given to appearance as with anything that goes onto a bicycle.
(Scratches, Nicks & Dings*)
*kcirick a ding is a small dint/dent usually the result of 2 bits of metal contacting each other quick, like if you were locking your bike and let it slip against the pole, or worse, when some ****** lets their bike ding against yours when they lock up.