why group by is used in sql code example

Example 1: sql group by

# Say you have a table called SALARIES that contains a 
# few duplicate NAME entries...
+----+-------------+--------+
| ID |    NAME     | SALARY |
+----+-------------+--------+
| 1	 |     Bob     |  500   |
| 2  |    Alice    |  500   |
| 3  |    Alice    |  200   |
| 4  |    Frank    |  700   |
| 5  |    Percy    |  100   |
| 6  |    Percy    |  800   |
| 7  |   Cyrille   |  400   |
+----+-------------+--------+

# We can obtain the total salaries of each person
# by using GROUP BY in the following query...
SELECT NAME, SALARY FROM SALARIES GROUP BY NAME;

# Which will output the following...
+------------+--------+
|   Alice    |  700   |
|    Bob     |  500   |
|  Cyrille   |  400   |
|   Frank    |  700   |
|   Percy    |  900   |
+------------+--------+

Example 2: group functions in sql

Multiple Row Functions (Group functions, Aggregate functions):
(Count, MIN , MAX, AVG, SUM)
will run for multiple rows and return a single value