The Kung Fu Tea Blog has a great post up about fighting styles as a martial brand. While it does focus on Wing Chun, the author’s argument applies to many other martial styles.
You cannot really understand why Wing Chun is a separate art just by looking at its techniques, which in truth are similar to other things in the region. Nor do its creation myths solve the mystery. Those are likely a product of a 1930s marketing campaign and are remarkably similar to other stories told in the Hung Gar and White Crane traditions.