Another use is for the young kits when first coming out of the nestbox. Their tiny feet and legs can go right thru the wire floor making them very unsteady. A resting board gives kits a safe place to explore and move around.
A third use is for bunnies to get away from drafts rising up underneath the cage. Especially important when housing them outdoors. This deflects the cold air off their entire underside, not just the feet.
You can buy pre-made plastic resting boards, use a piece of carpet or a smooth wooden board. Here's a tip - when garage sale shopping look for old cutting boards. Take them home and bleach them. Voila! Instant resting board.