One example: REPLs can occasionally be a great way to implement​ helper methods or other well isolated functionality. I understand you might argue unit tests are better for this, but I would very strongly disagree they are better in every situation.

Unit tests provide some additional benefits, but are a heavier approach with a slower, clunkier feedback loop. There are times when those additional benefits are not worth the added cost. Unit tests are also additional code that must be maintained.