When you plug an external hard drive plugged into your Mac, you might prefer to set the permissions so that the drive is only readable, to prevent people from adding files to it, but then later realize you need to make the drive writeable. For example, you have an external hard drive whose contents you intend to protect from being overwritten, or you want to provide read-only access to a portable drive that contains files you want to share. If you determine that you now need to write files to the drive, you can easily change its permissions.
