Child theme is created by you or someone else to be safe from your content loss while updating to new version. If you like to add css in your child theme then only add the required css for modification. Other default css will be loaded from your parent theme.
If there is nothing code customization then its not required to install child theme but if you have installed child theme then style.css is required but you only add the required css. Other than unwanted css you can remove it from your child theme.
Thank you!