But in a system that exists without tests (this is the real world after all), the current functionality is already enshrined in the app.

Adding tests that capture the current state of things, so that when that bug is uncovered tests can easily be updated to the correct functionality to prove the bug prior to fixing it is a much better place to be than the status quo.

The horse may have bolted from the barn, but we can at least close the farm gate in the hopes of recapturing it eventually.