Also used to filter wine - see https://www.winespectator.com/articles/are-egg-whites-really...
First time I heard about "Swedish egg coffee" from American colleagues I was really puzzled, had to do some research about it because I never heard or seen it. It seems to come from some Swedish immigrants to the USA that used eggs instead of fish skins to remove impurities from coffee.
I was skeptical but...it's actually great.