It's all a balance, but one thing to consider is that future maintenance or new development may require changes to that software. At that time, having good test

It's all a balance, but one thing to consider is that future maintenance or new development may require changes to that software. At that time, having good tests will help you avoid breaking things in subtle ways. The problem is that when you reach that point, nobody may remember enough about that code to write the tests, so it's preferable to have already written them. If a piece of software is trivial or isn't core to your business, though, obviously this doesn't apply.