Passing tests doesn't mean a working application

Having a passing test suite doesn't mean everything in your application is working.

It means the functionality you previously tested is working.

There may be edge cases you haven't covered or whole tests you haven't written yet, which may be working or could be broken.

If you have tests that were passing that are now failing, you know you've broken something.

Something that was previously working is broken and shouldn't be deployed.

That's why having tests is important - they give you the ability to identify and fix regressions before they are released.