I don't know if this is actually true, but I've heard the stripes make it difficult to track an individual zebra in the herd. They blend in with the other zebra making it harder to stay on the one that is being hunted.
From what I understand, there are multiple theories. One is that it confuses horseflies to make them less likely to bite. There's been some experimentation on that and it does seem to have that effect on them, but we don't know if that is why they evolved the stripes or if it's just an added benefit.
I think the point is that while this photograph kind of works (from above, at a distance), generally from ground level it doesn't. So if it only works from above, then is it really useful as a camouflage?