Slice makes the ball float farther in the air, so it's not a good choice when you want to hit hard and clear the net generously. That floating effect can help you get more depth when you can't hit as hard, though, such as when you're a little late on your stroke, and this makes it a good defensive spin. Slice also makes low shots skid lower on the bounce, which enhances approach shots, and it makes short shots stay shorter, which enhances drop shots.
Slice is used much less frequently on forehands than on backhands, primarily because hitting topspin is usually fairly easy on a forehand, and topspin offers more advantages for most situations. Grip permitting, most players can hit a forehand slice comfortably at a point of contact between a little below the knees to around the top of the head. The more Western grips are poorly suited to slicing at any height.
Slice makes an excellent approach shot, and a hard slice drive can be much easier to execute on a high, short ball than a topspin would. You'll also see some players use a slice shot sometimes called "the squash shot" in extreme defensive situations where they're stretching for a wide, low ball. It's always good to confuse your opponent a little, too, so throw in a few slices just to mix things up.
