That's an excellent question! Licenses are used to link devices to your digital account. If your device has been registered with a license, then that license is used up and the device is able to access any digital document that you have purchased or will purchase in the future.
You are given 1 license with your very first digital purchase. This allows you to access any future digital purchases on the one device. If you wish to access digital files on a different device, you must request a license for the device in order to be able to do so.
Licenses are not transferable, so make sure you register your first license to a device that you plan to continue using for years to come. Otherwise, you will need to request additional licenses to register additional devices.