def test_sum():
      assert sum(1, 2, 3) == 6
  def test_sum_no_args():
      assert sum() == 0
  def test_sum_negatives():
      assert sum(-1, -2) == -3

This means you only do the "REPL" testing once ever for each case - and you can rerun the tests for all future changes. If ever type the same function + args into the REPL more than once - you have done redundant work and are wasting your time.

Likewise if you're in the habit of writing tests after your function is made - it shows you haven't put much thought, which means you haven't done the proper analysis of why the function exists, if the function is really one function or should be two functions, what arguments it can take, what it should return etc.