The correct answer is: To counteract the forces of friction and wind resistance that oppose motion.
When an object is in motion, various forces can act on it that oppose its motion, including friction (especially if it's moving along a surface) and wind resistance (air resistance) if it's moving through the air. To maintain a constant speed, these opposing forces must be balanced by applying a constant force in the direction of motion. If these opposing forces are not countered, the object will eventually slow down and come to a stop due to friction and air resistance.