The band is called a stabilimentum, a name given when it was assumed to strengthen the web. It is a dense zigzag of white silk laid at the hub, and Argiope aurantia is among the species that build it most conspicuously.
The competing hypotheses are all plausible. The band may warn birds away from a web that would otherwise be destroyed on impact. It reflects ultraviolet, which some experiments show attracts insects that respond to bright open patches. It may break up the spider's outline, or advertise nothing at all and simply be surplus silk. Some studies find decorated webs catch more prey; others find they are found more often by predators and parasitoids.
The pattern of building complicates every version. Juveniles decorate more than adults, individuals vary from night to night, and many related species build nothing of the kind.