The stripes
Field experiments led by Tim Caro found that biting flies land far less often on striped surfaces than on plain ones, and that where zebras live overlaps closely with where tsetse and horse flies are worst. The older ideas, camouflage from lions and confusing predators, have not held up as well in tests.
Each animal's pattern is as individual as a fingerprint, and foals learn their mother's pattern in the first days of life.
The migration
Plains zebras travel with the wildebeest on the great Serengeti–Mara migration, often out in front, grazing down the tall grass so the wildebeest can reach the shorter growth behind them. It is the largest overland migration of mammals left on the planet.