@tillbtc
Welcome!
It's to share an image with little or no context (besides alt-text) and let the image provide some pause in your stream.

'Silent Sunday’ was first used by photographer Jay Mountford. She decided that every Sunday she would post a picture without any words - just to look at and not be distracted by words. See
mochabeaniemummy.com/silent-sunday/

Since then the hashtag use has changed a lot, but the sensibility is still there.