Is doing_wp_cron a necessary query string when scheduling cron.php as a cron job?

I have seen various ways to schedule the WordPress cron so that it is triggered by the server. Some will run the php file directory from the crontab and others will use a wget and reference the url. I have noticed some sites suggesting this:

wget -q -O - https://domain.com/wp-cron.php?doing_wp_cron /dev/null 21

What is the difference between this and just using

wget -q -O - https://domain.com/wp-cron.php /dev/null 21