This part of an insect's face is a sclerite (single piece of chitinous exoskeleton; essentially the equivalent of a bone) called the clypeus, and as far as I can tell, the ridges probably provide structural support for the muscles that engage the sucking action of the "rostrum," which is the name of the beak-like structure their mouthparts form that they use to feed on sap and other plant fluids. Rib-like structures such as this are often places where muscles attach to the inside of exoskeletons. This is just an educated guess, really, I've never dissected a cicada.

I'm not an entomologist, just neurodivergent and specially interested in insects.