The reason why it is convenient is that when you have a common library which is stable (or mostly stable and if it needs an update then all the services based on it need the update) and want to configure multiple services (i.e. create multiple instances) with different configuration.
So if you bundle the library with each deployment then:
- to modify the config you need to repackage;
- to upgrade the common library you'll need to repackage all the deployments including it.