A friend got us some before our TransAm, and I rode an ACA tour which supplied and required them. Unless the tour operator's requiring it, I figure they might be useful in fog, rain, or overcast. So are brightly colored shirts or jackets -- yellow, high-vis green or orange.
On one foggy, forested climb, I came up behind a guy who was running eight (8) rear flashers. I noticed them when I was about to pass him, but the green jacket he was wearing had caught my eye 4-5 times further back.