Why did the officer made me take the oath ?

So, I successfully immigrated to the United States. However, when I looked back at my immigrant visa interview at a US consulate abroad, I remembered the officer asking me to take the oath of allegiance. I was 17 at the time, so I basically inherited US citizenship from one of my parents. As a result, I didn’t have my green card for very long. But looking back, I wonder if that was the reason. Thank you.