I don’t get it, how can you truly add grain to something that has been degrained? I always thought once the grain has been scrubbed it’s done.
Film grain overlays, or video effects plugins like Boris FX Sapphire’s S_UltraGrain (which is in some ways more realistic-looking than grain overlays). It’s not really adding the removed grain back (which is impossible), but adding new grain in. In some cases, it can closely approximate the appearance of what the footage would have looked like if the grain had never been removed.