It was initially signed as a standalone agreement 150 years ago for the sake of reciprocity - rightsholders in different European countries were unhappy with pi

It was initially signed as a standalone agreement 150 years ago for the sake of reciprocity - rightsholders in different European countries were unhappy with pirate publishers over the border selling their works without payment. Today though pretty much everything in the Berne Convention is part of agreements you have to implement to join the World Trade Organisation, which is a pretty big motivation to abide by it.