I don't necessarily look at tests as something for the past, I look at them for the future. If I want to add a feature or change some code around to fix someth

I don't necessarily look at tests as something for the past, I look at them for the future. If I want to add a feature or change some code around to fix something (like, say, migrate from one giant file to a bunch of individual files), then I want some sort of assurance that I haven't broken anything. That's what tests give you. If you have someone contributing to your project, who doesn't necessarily have the whole 3,000 lines in their head, then tests give them the ability to make those contributions safely, without breaking anything. It is all about doing things as standard as possible. Lowest common denominator for every one looking at the code base. As soon as you start to do things strangely in code (like a big single file with no tests), then people start to curse you. "What were they thinking when they wrote this?!!#@#!@#". I try to avoid those situations. Tests also help document "the why". They become a history for changes over time. I could start to fix something that seems like a bug or strange design decision and look at the tests and realize that things were done intentionally... and give me the "why" they were done that way.