This public domain app, written in the python scripting language, allows you to download videos from YouTube. If you need to archive a video for various reasons, this utility makes it easy. However, this script requires a command prompt or terminal emulator, and so it may not be preferable for those who prefer a graphical user interface (GUI).
But how is this most useful? Imagine you uploaded some videos to YouTube, but you lost access to the account. You could contact YouTube support, but this could be a long and tedious process. With youtube-dl, you can archive your videos and save them locally, in the cloud, or you can re-upload them to another YouTube channel that you manage.
Likewise, if you find yourself in a situation in which you need to extract a clip from a video on YouTube to use in your own work, this script will help you accomplish the task—provided that your usage is compliant with fair use provisions.