The child themes are meant to be a safe way to keep customizations of the main template without fearing to lose them on a theme update.
Basically, whenever you want to edit a file inside the active template from your website you have to ask yourself "What is going to happen when I will update the theme?
And the answer is simple: You lose them because the update will bring an entirely new theme folder.
So let's look at a child theme as a folder with files which will overwrite the files with the same path in the parent theme. Now let's bring some real examples: