It’s mainly for videos that have been upscaled using standard filters. But also it’s for anything that’s out of focus—I just haven’t had great results with it for those kinds of videos.
i actually use it to ‘denoise’ very noisy 70s-90s movies (in addition to Proteus). the loss in detail due to the downscale is for that application not really relevant due to the source material not having much detail anyway. it’s very effective and fast for that purpose