It is most commonly used in weddings, when the bride gets decorated with motifs and designs as a wish for a happy life with her groom. Among the several motifs, floreal ones symbolize fertility and rebirth, and are then often used in weddings.
For more motifs and meanings you can check this page.
A design with such styled flowers was requested by Ashanti.
A curiosity: if the bride´s motifs were still visible after a few days on the palm of her hands, this meant that she was being treated good by the groom´s family, as she was clearly not doing any home work. It´s tradition thus not to let the bride do any home work until the mehndi has completely faded out.