Because:If you can get the cue ball to the next destination with a vertical axis hit, I see no reason to spin it. However, many shots require spin to get to the next destination, so, as BB said, you need to be able to do both-
- Sometimes the spin increases the margin of error for the shot, the position, or both
- Reduces the chance for skid
- To avoid scratching when cinching the case ball
- To not have to hit as hard
- To not have to hit so soft
Spin makes this game so much easier than trying to stay in the vertical center. You don't "have to" use spin, but it's actual easier overall. No question about it (even though people will inexplicably question it).