Maybe it's meant as a compromise for women who want their attendants to wear fairy wings but whose mothers are appalled by the idea? Or maybe it is a symbol to show that the bridesmaids are superheroes in training and anyone who wishes to interfere in the wedding is duly warned? Or a bunch of bridesmaids got together and said "look, we're tired of having to buy new bras to wear with these strappy dresses; we demand regal shoulder coverage!"