It's because, packagist automaticly creates versions of a package, so you can specify which version you want to install. Without tags you can only install dev-master version (current latest commit) which composer considers as development while tags considers as stable. With tags composer installs the latest tagged version if you don't specify any, but if there is none you have to explicitly specify dev-master if your composer json is not configured to accept it automatically. See my module https://github.com/uiii/ProcessWire-FieldtypePDF as example.