Black stripes

Hi guys,
I've been wondering why is the black stripes at the tail of surfboard tail, usually traction pad to be installed in between. I thought it's was marker pen lines initially. But when Iooked closer, it looks like tread. Not all surfboard has these but mostly has them. What's is it and the function of it?
I've been wondering why is the black stripes at the tail of surfboard tail, usually traction pad to be installed in between. I thought it's was marker pen lines initially. But when Iooked closer, it looks like tread. Not all surfboard has these but mostly has them. What's is it and the function of it?