Have used various video downloader tools for similar reasons others have listed.
Until recently, it was sometimes a necessity for offline presentations (ie. classroom), where the network or service could glitch or introduce ads on a campus-managed system where we could not install ad-blockers, etc. With downloaded files on USB device, instant access is there when any clip is needed during a discussion.
Also, over time it becomes increasingly more difficult to locate specific videos, or decent quality versions. Newer search code de-prioritizes content that is not being tapped by current “trends” or that are not promoting a product/service. Beyond the search problem, many useful, informative, entertaining videos that have vanished over the years for whatever reason. So even if you have the bookmarks for a URL, it may be gone.
Another application is having access to videos for offline environments (maintenance, repairs) where signal may not be ideal or available and having greater control of playback. Youtube, for example, introduces overlays and elements that even block important content in the video underneath (most commonly near the end where suggested videos pop up).