mysql select nested statement

Dear Experts,
I want to learn why I need to use NULL twice for this nested statement? what is NULL for in this statement?

SELECT table1.uid, 
       table1.isim, 
       NULL, 
       NULL 
FROM   table1 
WHERE  NOT EXISTS (SELECT 1 
                   FROM   table2 
                   WHERE  bayikodu = table1.uid); 

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