Björn Borg ushered in the forehand that almost every pro has today. Until he came around, almost all players had an Eastern forehand. Borg used a Western grip and hit the ball so far over the net, and with so much spin, that he rarely missed.
If you have an Eastern forehand,you can hit the ball flat, but in order to create topspin with that grip, you had to have a straight arm, which can cause tennis elbow.
The semi-Western grip can give you the topspin you need and still allow you to hit the ball flat. Borg’s consistency and ability to hit high over the net gave his shots an enormous margin for error. That’s a skill that club players need, and Borg was the master of it.