Whether you use a brand name stencil like "line em up," or your own steady hand, having a permanent line on your golf ball makes it easier to see the line you want the ball to take. It's particularly useful for putting; just point the line on your ball at the center of the pin and you know your putt is lined up. Then when standing over your ball to putt you will be less inclined to doubt your direction. You know that a well placed tap along your mark will send the ball toward the cup.
You can even use your black mark to line up a drive; it has the added benefit of encouraging you to keep you head down and eye on the ball.