SELECT * FROM comments WHERE parent_id = 2 || parent_id = 4

と

SELECT * FROM comments WHERE parent_id = 2
UNION ALL
SELECT
  comments.*
FROM
  comments JOIN (
    SELECT * FROM comments WHERE parent_id = 2
  ) AS t
ON
  comments.parent_id = t.id;

では得られる結果は同じになりますが、なぜこのように書くのでしょうか？