I recently started maintaining a decades old PHP application. I have been selectively placing in unit and functional tests as I go through the massive codebase. Whenever I refactor something I've touched before, I tend to get failing tests. If nothing else, this tells me that some other component I worked on previously is using this module and I should look into how these changes affect it.

Unfortunately, the existing architecture doesn't make dependencies obvious. So simply knowing that "something has changed" is very, very helpful.