Why is imperative mood important for docstrings?

From the docstring of check_imperative_mood itself:

  """D401: First line should be in imperative mood: 'Do', not 'Does'.

   [Docstring] prescribes the function or method's effect as a command:
    ("Do this", "Return that"), not as a description; e.g. don't write
    "Returns the pathname ...".

(We’ll ignore the irony that this docstring itself would fail the test.)