Why do we need a block? And where do we define it?

I think templates are flexible, and I use them as partials and "blocks".
Therefore I'm curious about how to use a block.

For example, I could do this (if I'm not wrong):

Views/index.ctp

<?php echo $this->element('someBlock'); ?>

Then in Views/Elements/someBlock.ctp

<div>
  //some layout
  <?php echo $this->element('someBlockContent'); ?>
</div>

Then finally in Views/Elements/someBlockContent.ctp

<div>
    <p>This is someBlockContent's content</p>
</div>

Nested templates works the same as a block?

In CakePHP 2.1, Blocks are a part of the core, and are used to display
content, scripts and css as a part of the layout.

You can start a block by simply writing to it, using any of the available
block commands (i.e. $this->start(), $this->assign(), $this->prepend(),
$this->append()), although your mileage may vary if you start appending to
a block that hasn't been created.

Your example is using trying to use nested elements, rather than blocks.

The strength of blocks is that you can write to them from helpers, layouts,
views and elements, and then choose to display the block at a later time
the view or layout processing.  

Several helpers could write to a block that is displayed by an element, and
element would not need to know about those helpers.  Without blocks, the
element would need to know about those helpers to display the content.

Regards
Reuben Helms

Thanks for the information.

Is there a good example where using a block is more sensible as compared to
template?

First, we should probably start by using the terminology that is used by
CakePHP.  We have layouts, views, elements and blocks.  Layouts, views and
elements all use CakePHP templates, as denoted by the extension "ctp".

As a result, blocks complement templates, whether it be layouts, views or
elements.  You could use a block in any of those.

The best example of block usage is the script, css and content blocks as
used by CakePHP itself. All output from the view is written to the content
block.  All css links that are not inline are written to the css block (and
this can be done from the layout, view or any elements included in the
view).  All script links that are not inline are written to the script
block.

One other example I could think of is an element being included in a view
or layout multiple times, with the same data.  If that element was
computationally expensive, you could process once in the view and out to a
block, then display that block in the element.  The processing only gets
done one.  Even better, you could have the element output to the block, if
it is empty, then output the block, Subsequent calls would just output the
block. There are probably alternate solutions using caching as well, but
that would be for when element content is the same across multiple calls to
the site, rather than just calls to the same element within a single view
call.