I like to use a piece of untreated shelving board cut to size. They chew on the edges, rounding it off. They are easy to clean if needed. If a rabbit decides to use it as a potty board than I can get them a plastic one sold for rabbits in the pet section somewhere.
It is good for their feet and to have a resting board and they like them.
It's a good practice to give rabbits a place where they can rest off the wire, even if cage floor is 1/2" x 1" as recommended. There are some who don't use them and select breeders based (partly) on foot quality. A rabbit with a thick, well-furred foot *shouldn't* have problems with foot sores if there is no resting pad.
I find a couple of my rabbits like to play with theirs. They chew on them, push them around the cages, etc.
We have a french lop buck who keeps pickin' his plastic one up and leaning it up on the wall of his cage!
I'll make him a wood one soon but but he has such a good time with the plastic one I think he'll have to have both!
Rabbits are such funny little creatures. I have a NZB buck that refuses to have a urine guard on the front of his cage. Doesn't matter how secure it is he manages to take it down. I finally let him have his own way because he choose the back of the pen as a bathroom.