At Phys.org:
Researchers at the University of Bristol have found why zebra fur is thinly striped and sharply outlined.
Their findings, published Feb. 17 in the Journal of Experimental Biology, reveal that stark black-white distinctions and small dark patches are particularly effective in thwarting horsefly attack. These characteristics specifically eliminate the outline of large monochrome dark patches that are attractive to horseflies at close distances.
The team theorizes that the thin back stripes serve to minimize the size of local features on a zebra that are appealing to the biting flies. – University of Bristol, February 20, 2023
This vid claims something else; that it’s actually camouflage:
The paper is open access.