I don't think it is very important except on the serve. The slice serve can be very effective and draw the opponent off the court. The lefty slice can also mess with you, if you don't get to play against lefties much.
On ground strokes, topspin is by far the most important spin, because it helps bring the ball down into the court, allowing you to hit harder and with more net clearance without going long. Side spin just makes a ball a little more difficult to return and can help make drop shots more effective by moving the ball farther from the opponent.
If you hit ground strokes you're probably also hitting some side spin whether you want to or not. It is usually easier to hit spin by pulling the racket toward the center of your body when hitting. The other famous side spin is the "banana" down-the-line running passing shot.
The standard side spin on the slice backhand is caused by pulling the racket in toward the center of the body. The "banana" shot is made by swinging the direction you're running and hooking the ball back into the court.