Started to dive into Drupal 8 twig, and what better place to start than from the top:  html.html.twig,  except.... wait, wtf?  


    <head-placeholder token="{{ placeholder_token|raw }}">

I was able to track down the related issues: 

* https://www.drupal.org/node/2565675
* https://www.drupal.org/node/2538950
* https://www.drupal.org/node/2603074

But nothing really clarifies what is going on here.

Is this a Drupalism? A Twigism? What are these tokens for?  

The only other time I've seen a placeholder token system like this was with CTools where they create a placeholder in the generated HTML output and then go back later and replace the token with the actual value.  If this is the same concept, why does Drupal core need to do this? (The CTools system was ostensibly to get around some core limitations).