The first movement in the golf swing plays a large role in determining everything else that happens. Just as the proper setup puts you in the right position physically, the right takeaway puts your swing in the right position.
There is a particular arc your swing would travel through if you were to hit the ball with maximum efficiency. Only if you start the club off along that arc can it continue along that arc. Granted, the takeaway only governs the backswing. But the purpose of the backswing is to get the club in the right position for the downswing. The right downswing only comes out of the right position at the top of the backswing, which is governed by its very first movement–the takeaway.
By “takeaway,” I mean the first few feet of the backswing, before the club head has even gotten to the height of your hip, before your body has even started turning. Just that small movement should feel like is moving along along the line extended behind the ball along which you want the ball to start off. By doing this, you program your swing to return along this line through impact. You give your unconscious mind the directions it needs to bring the club back through the ball along the starting line.
Now there are two things about this that are true. First, the swing is an arc, and has no straight lines in it. When the clubhead meets the ball at impact, it might be traveling along a line that is straight for only a few inches at best. But there is an overall orientation of the arc that must be parallel to the starting line. If we extend the swing arc to make a complete circle, like a giant hula hoop, a line running across the center of the hoop, parallel to the ground, would be parallel to the starting line, too.
This is what we are trying to achieve with our swing, and starting the club straight back makes this much easier to do than otherwise.
The second thing that is true, is that many top-flight golfers do not take the club straight back. Many of them take the club back inside of this straight line and come back to the ball swinging from the inside to out. These golfers have, however, learned how to compensate for this and still hit marvelous shots.
As recreational golfers, without the time available to learn compensations, our best strategy is to learn how to take the club back straight.
The best place to learn how to do this is in your kitchen, since that room is the least cluttered with furniture, allowing you to swing the club back a few feet without hitting anything.
Take a golf stance with the toe of your club up against the baseboard. Now take the club away as if to start a golf swing. (Be in no hurry doing this. Clubhead speed gets built up on the downswing, not the backswing.) You should hear a brief scratching sound, like striking a match. That is the sound of the club moving straight back against the baseboard for a few inches before your body turn arcs the club inward.
If you hear no sound at all, you’re taking the club back inside. If you feel like you are pushing against the baseboard when you take the club back, then you are trying to take it back outside. If your kitchen is not appropriate for this exercise, go outside and rest the toe or your club against a block of wood, at last a foot long, which is firmly supported so your club doesn’t push it out of the way.