In low traffic/back roads on my bike, I can tell if there's a car coming up behind me without having to twist around. Doesn't work if there's other cars drowning it out, and I definitely wouldn't recommend relying on that in most cases, but it's neat.
Bicycles and e-bikes are great. Would love to have a high powered e-bikes/very small e-motorcycle that could go on the freeway (ie., be able to cross the bay bridge. Have yet to see such a thing is America.