A full follow-through takes the bowling arms from its release point on an upswing to shoulder lever or higher. The line of the follow-through should be toward the target, not left or right. Although the ball is already gone when it occurs, practicing a complete follow-through is a great training aid. Its improves the function of the swing and overall approach balance. If you are going to toss a ball at something, normally you look at the target.
Pitchers watch their catcher's glove.
Basketball players shoot at a basket. Quarterbacks, unless they are deceptive, look at their receiver. Granted, the ideas is to get the ball rolling toward specific pins. It is also easier to see and make a connection to a marker that is much closer. The arrows, or rangefinders are located 15 feet from the foul line.