Странный код для LAG курс 2 тема 6

WITH users_cnt AS
   SELECT session_start::date AS session_date,
          COUNT(DISTINCT user_id) AS users_cnt
   FROM online_store.sessions
   GROUP BY session_date
)
SELECT *,
      LAG(users_cnt, 7) OVER (ORDER BY session_date) AS previous_weekday_users_cnt
FROM users_cnt;