It is crucial that each piece of code you deploy have a unique version, it helps you track what your client is running, mark deployments with breaking changes, and makes your life so much easier — especially when trying to understand what changes are running at your client site in the middle of the night.
When you develop microservices, it is twice as important. Usually, you deploy your services individually and you need to know which version of which dependency is used by your service. Another requirement many developers face is how to automatically (or manually) update your dependencies when they change.