How does the MySQL GROUP BY clause behave like a DISTINCT clause?

mysql> Select * from testing;
+------+---------+---------+
| id   | fname   | Lname   |
+------+---------+---------+
|  200 | Raman   | Kumar   |
|  201 | Sahil   | Bhalla  |
|  202 | Gaurav  | NULL    |
|  203 | Aarav   | NULL    |
|  204 | Harshit | Khurana |
|  205 | Rahul   | NULL    |
|  206 | Piyush  | Kohli   |
|  207 | Lovkesh | NULL    |
|  208 | Gaurav  | Kumar   |
|  209 | Raman  | Kumar    |
+------+---------+---------+
10 rows in set (0.00 sec)

mysql> select Distinct LNAME from testing;
+---------+
| LNAME   |
+---------+
| Kumar   |
| Bhalla  |
| NULL    |
| Khurana |
| Kohli   |
+---------+
5 rows in set (0.00 sec)

mysql> Select LNAME from testing GROUP BY Lname;
+---------+
| LNAME   |
+---------+
| NULL    |
| Bhalla  |
| Khurana |
| Kohli   |
| Kumar   |
+---------+
5 rows in set (0.04 sec)