Like his name and what most of people explain, you archive videos and keep them locally whatever happens to them online.

But, personnaly, I use it mostly to watch videos with a better latency and almost no loading time, and I feel TubeArchivist very confortable to use (I use FreeTube as well, for less important channels) as you can share play status between clients.

I only keep videos I want to archive. I delete most of them.