Leather armour may not have been as popular in life as it appears in entertainment. It won't prevent a sword going right through or an axe from dealing a fatal blow, but it will muffle most swipes or slashes and can cushion the wearer from projectiles fired from a distance. Leather had many positive factors going for it such as being relatively cheap to make, more mobile than a plate, more durable than cloth, and troops could provide it themselves. One downside is that it was prone to rot and therefore would need to be covered in grease, pitch, lacquer, or possibly even mutton-fat as a weather-proofer. On its own, it was certainly better than nothing, but it's use increased dramatically when paired with equally free moving chain mail.