It is easier to read: When is what called? If you are debugging a hook you can immediately see if you have to read the function or not: If it is not your hook, you can skip the code.
In my themes and plugins I combine all registrations for actions, filters and shortcodes at the top and I add the hook to the PHPDoc block:
add_action( 'wp_head', 'foo' );
add_action( 'shutdown', 'bar' );
* Foo you!
* @wp-hook wp_head
* @return void
print '<!-- foo -->';
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