Should I use parent or child theme in WordPress?

Parent and child theme is a nice and needy concept added by Wordpress. Sometimes, out of the box themes do not work and we need to add custom code/widgets to change functionality or introduce a new. So this works, but when we update the theme all changes get lost, as we done the change in theme files.
So, to prevent the change, we use child theme. Child theme is a minimal config files copied from main/parent theme files. This does not contain all files, but only minimal required. This way, we can use parent theme for other files, without affecting the changes and preserving child theme changes. We can add more config to customise more from parent theme for whatever changes we require.

So, parent child theme helps
1. Custom changes
2. updating themes
3. preserving changes.

Thanks