Marlon Brando knew it. So did Joan Jett and Kate Moss. A well-chosen piece of leather has always been an easy way to bring a little rock-’n’-roll cool to any ensemble: throw on a biker jacket with jeans and a T-shirt or over a dress, and you’re ready—complete with a look that says, "Nobody tells me what to do." But this season leather invites you to discover its more sophisticated side, appearing on runways not just in iconic motorcycle jackets but also trimming trench co...