There are actually three segments to the golf swing. The segments are the backswing, the downswing and finally the impact and follow through.
Dividing the swing into these 3 segments helps to apply every individual golf swing instruction or tip to the particular segments where they apply. It also helps the golfer focus on one segment at a time in their golf swing instruction making it easier for them to remember what they learn and also to be able to work on improving their game systematically.
There is another bigger advantage to this approach. And that is the ability to be able to draw ones attention to the many different parts of the body and the many muscles involved in a proper golf swing.
This is important in helping a golfer involved in a golf exercise program to be able to appreciate the different exercises they do in the program and what particular muscles the exercises focus on.
All this makes it a lot easier for any golfer to link their exercises to golf swing instructions they receive as they seek to improve on their game.
Actually it is not possible to divorce the basics of golf swing instruction from golf specific exercises. The two go together and chances of improving your game improve dramatically when you combine the two. On the other hand progress is bound to be much slower when you work on golf swing instruction only.