why use union when left joining to a generated number also works, PostgreSQL

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create table test_topups (amount int, result varchar(30), user_id int)\\
insert into test_topups (amount, result, user_id) values
(10, 'success', 1),
(20, 'failed', 2)\\

create table test_withdraws (w_amount int, user_id int)\\
insert into test_withdraws (w_amount, user_id) values (10, 1), (20, 2), (30, 10)\\

create table test_transfers (method varchar(30), amount int, user_id int)\\
insert into test_transfers (method, amount, user_id) values
('abc', 10, 3),
('xyz', 20, 4)\\

create table test_users (id int, f_name varchar(30), l_name varchar(30))\\
insert into test_users (id, f_name, l_name) values 
(1, 'first1', 'last1'),
(2, 'first2', 'last2'),
(3, 'first3', 'last3'),
(4, 'first4', 'last4'),
(5, 'first5', 'last5'),
(6, 'first6', 'last6')\\

SELECT 
 tup.amount, tup.result, 
 usr.id as user_id, 
 wd.w_amount, 
 trans.method
FROM test_users usr
CROSS JOIN (SELECT generate_series n FROM generate_series(1, 3)) AS nr
LEFT JOIN test_topups tup ON tup.user_id = usr.id AND nr.n = 1
LEFT JOIN test_withdraws wd ON wd.user_id = usr.id AND nr.n = 2
LEFT JOIN test_transfers trans ON trans.user_id = usr.id AND nr.n = 3
WHERE (tup.user_id IS NOT NULL OR wd.user_id IS NOT NULL OR trans.user_id IS NOT NULL)
ORDER BY tup.user_id, wd.user_id, trans.user_id


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