Return value of add_menu_page

One important use is enqueue script / style only on specific plugin/admin pages.

<?php
    add_action('init', 'my_plugin_admin_page');

    function my_plugin_admin_page()
    {
        //create the menu page
        $hook = add_menu_page(....);

        //use the hook for this page for enqueuing
        add_action('admin_print_styles-' . $hook, 'my_plugin_admin_styles');
    }

    function my_plugin_admin_styles()
    {
        //enqueue the style/script here
    }
?>

This is one use I know.