Why is '..' used? and is '..' really useful? in the examples in the sections Directives, Images, etc

[hyperkai]

The sections Directives, Images, etc show the examples with '..' as shown below but I don't know why '..' is used and I even think '..' is just noise and useless so why is '..' used? and is '..' really useful?:

↓↓
.. function:: foo(x)
              foo(y, z)
   :module: some.module.name

   Return a line of text input from the user.

↓↓
.. code-block::
   :caption: A cool example

       The output of this line starts with four spaces.
↓↓
.. code-block::

       The output of this line has no spaces at the beginning.

↓↓
.. image:: gnu.png
   (options)

↓↓
.. image:: gnu.*

[timhoffm]

This is just how reStructuredText has defined the directive syntax. Directives are block elements and the double dot is the marker that a directive starts.