This is probably one of the biggest reasons One Piece's world is as fleshed out as it is. It basically allows Oda to flesh out side characters and side stories without taking away from the story he's currently telling. On top of that sometimes they converge into the main plot.
Plus they're pretty easy reads considering that its just a single image showing a moment taking place.
I just think its sorta weird that One Piece is apparently the only manga or maybe even comic to even do this. You'd think other people would be all over this, it seems so useful. Any explanations for why?