Hitting a slice ball is the same as using underspin.
A well hit slice can be extremely effective. When you slice well the ball will cut through the court and skid on contact with the ground.
The slice can be very effective for returning hard hit serves such as a flat serve or a hard hit kick server.
The slice can be used to attack with a chip-and-charge strategy and the slice can also be used as a defensive lob.
Developing your slice will also allow you to volley well.
If you need to see the slice in action, just watch matches with Roger Federer, he will often employ his slice to disrupt his opponent.
Another player to watch is Monica Niculescu, she hits a ton of forehand slices as that shot is very natural to her.