I'd argue that tests can also be a tool for improving your architecture (e.g. separation of concerns) and for communicating intend and usage. Tests can be a to

I'd argue that tests can also be a tool for improving your architecture (e.g. separation of concerns) and for communicating intend and usage. Tests can be a tool for documenting expected behaviour, intended use, and edge cases. In contrast to written documentation, they have to be up-to-date by their very nature, which is an advantage over code comments or external docs.