That's a really interesting question!
That said, I have no clue what the real answer is. :lol:
From what I've read, the dew claw can help dogs grip things, so maybe it helps rabbits as well. Also, there are theories that the dew claw on dogs touches the ground when they run, giving better traction. Once again, not sure about rabbits.
But those are my two theories! Gripping and traction.