I've never actually seen anyone putting them up, but I had vaguely wondered about them. I'd envisaged, perhaps, a pair of mourning parents, or a desolate deserted Significant Other, perhaps supported by a friend.
Today I saw the flower bringers. There were about 10 of them, all men in their early 20's, and rather tanned, all on foot, faces very serious, walking along a roadside in a loose group, with the two people who were holding bouquets at the front. As I drove past them, they arrived at a road sign and began to tie the flowers to it. I almost stopped and took a photo of them, but didn't quite have the nerve.