Interop with C/JS: you can't control all identifiers, because you can reuse a rich ecosystem of other language libs: you can't force their names to change
EDIT: you can still change them in importc pragmas, but this still seems less nice to me.
Still, you can reuse Nim libs even if they don't follow nep1: one can still use nimpretty/gofmt for his own codebase.