A child theme works within the functionality of a parent theme. You have to activate the child theme as well as you will customize the child theme also. The benefit is- you are not editing the core theme (Parent), instead customizing the child theme. As a result, the customization will not affect the core theme. The same thing will affect when you will update the theme. You will need to update the parent theme. That means, there will be no impact on the customization of the child theme.
For customization the child theme will get priority over the parent theme, and regular update will occur on the parent theme. So, both themes are safe always. Using the Parent and Child theme (wp-content/themes/) within WordPress is the safest way to extend the functionality.