Comment by ssalka

I forget where I heard this, but early in my career someone described unit tests to me as "a contract between you and your code." Which seems largely true – when I write a test, I'm saying "this is how a given function should behave, and that contract should hold true over time." If my future self wants the code to behave differently, so be it, but the contract needs to be amended so that the new code changes are also in agreement with it.

Conversely, if you fail to write a unit test, there is no contract, and the code can freely diverge over time from what you think it ought to be doing.