Part of engineering culture in general, not just software engineering, is ability to share knowledge though documentation and best practices. If you share the source code, you communicate your solution on a very low level. It is often necessary to zoom out to see the big picture and there visualization helps. You can create ad-hoc diagrams, but their expressive power is low: without a convention it’s basically just space, text, arrows and generic shapes. To increase the expressive power you need a visual language in which different shapes and lines have some semantics. And here comes UML and other diagram languages. If you are not using them, it is likely that you are not communicating efficiently. Can a mature team fail at communication? I leave the answer to you.