2 of 2 I typed cpt name wrong (not plural)

Why does a custom post type need the ''hierarchical' args setting?

I'm struggling with understanding some things about CPTs and and could do with some help. I can see that taxonomies often need to be hierarchical, but can't see how/when a custom post type would be?

i.e I have a CPT called 'projects'. There is a taxonomy registered for this posttype called 'fields'. Under 'fields' I have lots of sub-items like:

  • 'Documentaries'
  • 'Documentaries' > 'Self Commisions'
  • 'Documentaries' > 'Classic'
  • 'Corporate'
  • 'Corporate' > 'Digital Marketing'
  • etc...

So to view a project in 'Documentaries' > 'Classic' I use this URL: mydomain.com/field/classic/

This is where I actually have about 30 questions :) but will just ask the two most pressing:

  1. Why and how would the 'projects' CPT ever be hierarchical?
  2. Is it possible to view items in the 'classic' field like so: mydomain.com/projects/classic (mydomain.com/projects shows all items, but drilling any further breaks it)

Any and all help appreciated, I've spent about 3 hours playing with this and reading and can't get my head around it!

Thanks!

Below is my (cropped) code for reference:

function register_cpt_projects() {
$labels = array(
    'name' => _x('Projects', 'projects'),
    // all other labels etc..
);

$args = array(
    'labels' => $labels,
    'hierarchical' => true,
    'supports' => array('title', 'editor', 'author', 'thumbnail'),
    'taxonomies' => array('field'),
    'public' => true,
    'show_ui' => true,
    'show_in_menu' => true,
    'show_in_nav_menus' => true,
    'publicly_queryable' => true,
    'exclude_from_search' => false,
    'has_archive' => true,
    'query_var' => true,
    'can_export' => true,
    'rewrite' => true,
    'capability_type' => 'post'
);
register_post_type('projects', $args);
}

add_action('init', 'register_cpt_projects');

function create_projects_tax(){
    register_taxonomy('field', 'projects', array(
       'hierarchical' => true,
       'label' => 'Fields'
    ));
}
add_action('init', 'create_projects_tax');