The forehand grip works well for overhead forehands that are level with your body.
This change in angle is necessary in order to hit the shuttle straight. If you don’t adjust your grip, then the shuttle will go out at the side.
The thumb position
When you use a backhand or thumb grip for forehands, you should normally avoid having a big gap between the thumb and the racket handle.
Instead, allow the thumb to rest along the badminton racket handle; this will free up your wrist for more natural forehand movements. Think of this as adjusting the angle of the forehand grip: the angle is more like the thumb grip, but the finger placements remain similar to the forehand grip.