The most important reasons to use child theme instead of modifying the parent one:
- if we are to make changes directly in the parent theme files then while updating the theme to a new version the changes will be lost. Either we have to carefully carry changes in the new theme version before the upgrade.
- child theme in most cases allows to save changes while updating the main theme. It is easier to control updated changes, since include only those files that are subject to change is enough. If an error occurs it is always possible to compare the code with the original file. Development of child theme is a good start for the development skills in creation full-fledged WordPress themes.