Basically the use of SET SERVEROUTPUT is to display the query answer in SQL *PLUS interface...
When you use the DBMS_OUTPUT.PUT_LINE procedure, the procedure will write the passing string into the Oracle buffer. In order to print the content of the Oracle buffer, you should use the SET SERVEROUTPUT command to display the content of the Oracle buffer into your screen. You can also increase the size of the buffer.
Use the “Set serveroutput on” to display the buffer used by dbms_output.