Why using the first SELECT in the solution

I ran the following query and both the solution's and this query seem to result in the same set, could anyone explain why is there the first SELECT * FROM in the solution?

SELECT

e.emp_no,

e.first_name,

e.last_name,

NULL AS dept_no,

NULL AS from_date

FROM

employees e

WHERE

last_name = 'Denis' UNION SELECT

NULL AS emp_no,

NULL AS first_name,

NULL AS last_name,

dm.dept_no,

dm.from_date

FROM

dept_manager dm