not sure i agree with the premise that azarenka has less variety than sloane, anyway...
with more variety, you have more strategic and tactical options. this is important as your opponent will adapt to your style. just having an additional well-developed stroke gives you a lot of strategic options.
e.g. having a low, almost skidding, slice backhand is a great complement to otherwise top-spiny ground-strokes.
How does this work?
So, let's say you can have a grinding baseline game that focuses on topspin ground strokes. It all works well until you come across a player than can handle your high kicking shots as he is quite tall and is consistent enough to hang with you in rallies. Having the variety of a functional slice backhand gives you:
- the ability to disrupt your opponent's rhythm in rallies. Junkballing high (topspin) and low (slice).
- a deep and safe defensive shot if you are pressured.
- an approach shot.
Used at the right times, this could be enough to make your baseline game less predictable and force some errors from your opponent. All for the price of one stroke. What if you had a drop shot too?
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM