There’s a few spiders that do this (I know some orb-weavers do) and IIRC scientists don’t know for sure why, though it’s been hypothesized that it could be either to confuse potential predators or to make themselves and their webs obvious to larger animals.

Some orb weavers like Argiopes even make a thick zigzag construction in their web called a stabilimentum that we don’t really understand the function of, but one theory is to make the web more visually obvious so larger animals don’t accidentally barrel into them.

Spiders are neat.