Case Statement Assignment Task 2

/*Extract a dataset containing the following information about the managers: employee number, first name, and last name. Add two columns at the end – one showing the difference between the maximum and minimum salary of that employee, and another one saying whether this salary raise was higher than $30,000 or NOT.*/

#My code when run produces a single row output.Can't figure out why..The solution provided in the course makes use of a group by clause. Can I know why a group by clause was a requisite in this case?
#here is my code

SELECT
    dm.emp_no,
    e.first_name,
    e.last_name,
    MAX(s.salary) - MIN(s.salary) AS salaryDifference,
    CASE
        WHEN MAX(s.salary) - MIN(s.salary) > 30000 THEN 'raised by more than 30k'
        ELSE 'not raised by higher than 30k'
    END AS salaryState
FROM
    dept_manager dm
        JOIN
    employees e ON dm.emp_no = e.emp_no
        JOIN
    salaries s ON e.emp_no = s.emp_no;