Content disappearing is a big reason, but it's also great for channels that post b-roll or green screen videos/effects/etc if you like video editing.

Sure, you can grab them manually, or you can just have your folders of source materials available at the speeds of your local network, already stacked up and ready to work with.

Also, as someone in the infosec industry, a lot of conference talks and guides and useful videos get blown away whenever YouTube goes on a "hacking tutorial purge". This has already killed multiple channels full of useful guides.

Plus, YouTube has reencoded old videos and won't let you access the originals pre automated upscaling. YT is not a backup or an archival site. So, if that's something you want, you have to DIY it.