There are many good reasons to learn a one-handed slice for a backhand. Most involve the simple physical benefit of extra reach.
I learned it just to solve the problem of reaching balls that were seemingly out of reach on the backhand. It is also very energy efficient when you're losing steam. Rafa does it now to save the running, extend his reach, and save an unnecessary deep knee bend on a short or wide low ball versus using a two-hander.
Learn slice early and you'll find the flexibility of having it available will make you a much more challenging opponent. Additionally, the more spin control you develop the more likely you'll manage it better on your serve, volleys, lobs, and drop shots.
