Verbs with a definite object often get a prefix while verbs with an indefinite object usually don't get one. "Végre megvettük az autót." = Finally we bought the car. "Végre vettünk egy autót." = Finally we bought a car.
In this case, "meg-" doesn't change the meaning of the sentence. In other cases it can indicate that the action described by the verb has ended: "Megettem a csokit." = "I ate the chocolate (and nothing's left for you)."