Hello,

The Child Themes are basically used on the Site when you want to do the custom code changes at the Theme files level so that the changes won’t get wiped out when you update the Theme (if update gets released of the Theme).

So, if you have done any Theme file level change in your Parent Theme then you need to do that change in the Child Theme and remove it from your Parent Theme because when you update the Theme (which would certainly be Parent one), the Theme file level changes will get wipe out.

Hope we have cleared your doubts now.

Further, you can read more about it here on the following Articles, if they are of any help:

https://developer.wordpress.org/themes/advanced-topics/child-themes/
https://www.wpbeginner.com/glossary/child-theme/

Best Regards