What is this function for? In what circumstances would it be needed to change the thumbnail of a picture/video that we upload? I understand why it would be handy to change the thumbnail of an album, so we can choose a thumbnail that better represents an album. But a thumbnail that represents a single picture/video, is supposed to represent (and therefor resemble)... this single picture/video, so what would be a reason to change that?
There must be a good reason I am missing, otherwise it would not be put in I guess.