There are multiple good reasons:
- Allows more angles and directions when setting
- It helps the attacker to align himself better to the ball and the opponents field
- Surprises the opponent's mid-blocker due to less predictable play
- The setter is higher. That means that the way of the ball from the setter to the attacker is shorter
- If the setter is frontrow he is able to score directly on second.
- The setter has a wider range of the timing when he is setting the ball.
- The receivers can put the ball for a perfect reception closer to the net