get user role from user id and blog id

As a followup here is what I got to work:

<?php
$user_sites = get_blogs_of_user($user_id);

if ($user_sites) {
    echo '<p><strong>Current list of sites for this user:</strong></p>';

    foreach ($user_sites as $user_site) {

        echo $user_site->blogname;
        switch_to_blog($user_site->userblog_id);
        $theuser = new WP_User($user_id, $user_site->userblog_id);
        if (!empty($theuser->roles) && is_array($theuser->roles)) {
            foreach ($theuser->roles as $role) {
                echo ' - ' . $role;
            }
        }
        restore_current_blog();
    }
}
?>

I don’t know why it is necessary to use switch_to_blog when the blog id is used with WP_User but it only returns the main blog role in a multisite blog otherwise