This seems to be due to Google's new grouping of permissions for Android apps. The permission group you're referring to is described as "Uses one or more of: files on the device such as images, videos, or audio, the device's external storage". That means that if the app wants to use some of its own data which is located on external storage (which is pretty common), you're also giving it permission to access your media files on the device, because Google thinks (for some incomprehensible reason) that these are similar permissions and grouping them makes it easier for users to understand what's going on.