Tests won't tell you if your code works

Having a passing test suite or CI pipeline doesn't tell you if your application works.

There could be scenarios or edge-cases that aren't covered within the test suite and contain bugs, but aren't covered by the test suite.

There could be untested code that isn't covered at all.

A passing test suite proves that the tests that have been written so far pass and that there are no regressions introduced by the latest change.

Here's the thing

Instead of telling you that your application works by passing, the test suite tells you something is broken when it fails.

If a previously-passing test is failing, the application is broken and should not be deployed - not that the application is working if the tests don't fail.