Comment Walker vs. Comment Callback

We could rewrite:

wp_list_comments( array(
    'callback'      => 'bootstrap_comment_callback',
 ));

with the null walker parameter:

 wp_list_comments( array(
    'walker'        => null,
    'callback'      => 'bootstrap_comment_callback',
 ));

which means we are using the default Walker_Comment class:

  wp_list_comments( array(
    'walker'        => new Walker_Comment,
    'callback'      => 'bootstrap_comment_callback',
 ));

The Walker_Comment::start_el() method is just a wrapper for one of these protected methods:

Walker_Comment::comment()
Walker_Comment::html5_comment()
Walker_Comment::ping()

that, depending on the context, append each comment to the output string when walking along the comment tree.

Using a custom walker class, that extends the Walker_Comment class, gives us the ability to override these public methods:

Walker_Comment::start_el()
Walker_Comment::end_el()
Walker_Comment::start_lvl()
Walker_Comment::end_lvl()
Walker_Comment::display_element()

in addition to the protected ones above.

If we only need to modify the output of the start_el() method, we would only need to use the callback parameter in wp_list_comments().