1. To understand the cause of the outside-to-inside swing. With an outside-to-inside swing path, if your clubface is open, you have a pull slice. If sometimes you square the clubface to your swing path, you will have a straight pull. Not knowing whether you'll wind up with a pull or a slice can cause havoc with your game. Look at your divots or marks on the practice mat to see the evidence of your actual swing path. Make sure you maintain your normal weight shift and leg action. Watch your left knee and make sure it stays bent. Swinging with just your shoulders and arms encourages an abnormal swing path that can cause a pull slice. This drill will immediately tell you if you swing outside-to-inside.
2. To develop a repeatable swing from the inside. Practicing an inside-to-outside swing path with a headcover near the ball gives you a forceful visual key to remind you not to swing outside in. Position the ball near the front edge of the headcover to make the headcover most effective in protecting from an outside-to-inside swing. When you.re on a tee, position your ball next to a divot or mark. The position of the divot just outside your ball will provide you with a visual substitute and reminder of the headcover drill. Some golfers find it helps to elevate the left heel a little on the backswing. Planting the left heel on the downswing creates a lateral movement making the inside-to-outside swing easier.
3. To reduce off-center hits. This drill will also help you to hit the ball on the sweetspot by almost forcing you to hit the ball in the middle of the clubface.
4. To understand the importance clubface position. If your swing path is straight and your clubface is open you will still slice! That's OK for the moment because the next step will be to work on keeping the clubface square. Don't start aiming to the left to compensate or you'll get in deep trouble. Review your fundamentals again, especially your grip.

Do.s and Don.ts
Do this drill whenever you find yourself on the course waiting for a shot. Put a leaf, or something similar, just outside your practice swing and groove your swing path. On the practice range some golfers find putting two golf balls down and hitting the inside one works well for them. Some golfers mentally put a headcover down outside the ball on every shot. This helps them remember to focus on repeating the same consistent thoughts and actions regarding the swingpath. Try it. It can give you a mental reminder of the proper swing path on each shot.