What is the first name in the household registration book for?

Some people used other names when they were children. I changed it when I got my ID card. (For example, when I was a child, I had a three-character o(∩_∩)o, and later when I applied for an ID card, I changed it to match the name in the household registration book. However, some of my primary and junior high school graduation documents are old names) or some people listened to fortune telling and changed their names. This will also have a different name. It is only a file management time to fill in, which has no practical effect.