wanted to find why you joined to the tables again in your solution?

Here is the query which I am referring to , I  am getting the solution even without joining film_category and category tables to film_revenue CTE, can you explain why you joined Again.

WITH film_revenue AS (
  SELECT  
    F.film_id,    
    MAX(C.name) AS category,
    SUM(P.amount) revenue
  FROM payment P
  INNER JOIN rental R
  ON R.rental_id = P.rental_id
  INNER JOIN inventory I
  ON I.inventory_id = R.inventory_id  
  INNER JOIN film F
  ON F.film_id = I.film_id
  INNER JOIN film_category FC
  ON FC.film_id = F.film_id
  INNER JOIN category C
  ON C.category_id = FC.category_id
  GROUP BY F.film_id
)

SELECT * FROM (
  SELECT  
    category,
    FR.film_id,
    revenue,    
    ROW_NUMBER() OVER(PARTITION BY category ORDER BY revenue DESC) row_num    
  FROM film_revenue FR
  INNER JOIN film_category FC
  ON FC.film_id = FR.film_id
  INNER JOIN category C
  ON C.category_id = FC.category_id
) X
WHERE row_num <= 2;