By order of FAR Part 91.113, "vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft." It all starts before you leave the ground. You may be cleared for takeoff or taxi onto the runway to "line up and wait," but it's worth taking a final look around. You can't guarantee the coast is clear just because the controller told you so. There could be an airplane approaching on base or final that has not contacted the tower either because the airplane has lost electrical power or for other reasons. While this is unlikely, the consequences could be a midair collision, so it is worth it to visually confirm that nobody is there.
If you are at an uncontrolled field, that final check is even more critical. While it is highly recommended to communicate at uncontrolled airports, it is not legally required. The announcement you make before entering the active runway may not be heard by everybody in the area since some aircraft are not equipped with two-way radio equipment. And somebody could appear unannounced.
So turn your head and look at the final approach path just like you would look in your blind spot before changing lanes on the freeway. There may be an airplane on its way in and, if you or the controller missed it before, this is your last chance to catch it. You may be in the habit of calling out "lights, camera, action" as you roll out onto the runway. Add to that "final is clear," and make the visual confirmation.
