SQL & PL/SQL

It was proposed to me that instead of joining 2 tables, it could increase performance in some cases to union them together and sum their results.

--NORMAL JOIN

SELECT a.emp_no, a.dept_no, b.salary
FROM employee a, payroll b
WHERE a.emp_no = b.emp_no
;

--UNION ALL and SUM

SELECT emp_no, SUM(dept_no), SUM(salary)
FROM (
   SELECT emp_no, dept_no, 0 salary
   FROM employee
   UNION ALL
   SELECT 0, 0, salary
   FROM payroll
   )
GROUP BY emp_no
;

I was given only anecdotal evidence for this method improving performance, but it does not seem completely unbelievable. In what case would one of these methods be preferable to the other?