The arms act like a hinge in order to keep the club in the correct swing plane.  Keeping it close to the body means that the body is going to work in harmony with the arms, not against them.  When you’re too closed during the backswing the swing is then too upright, so keep the dominant elbow close to the opposing elbow.  If the club is too steep, the elbow is probably not close enough.

If you see a picture of a golfer with a huge gap between elbows, you’ll notice quickly how nothing in their swing is working together – the body goes against the arms as they don’t hinge properly.  The body goes through something of a reverse pivot, leaning forward at the top of the backswing.  Often in cases like this their swing resembles that of a baseball player, not a golfer.  They will seem as if they’re smacking the ball hard, not swinging through smoothly.

Try to also pay attention to professional golfers and notice how their elbows are aligned.  Everything on their body seems to be working in harmony, not working against each other.  The elbows are crucial to this point.  When they’re out of alignment, the entire swing is off balance.  A few short practice swings will confirm this for most amateurs.