The basic idea
A child theme inherits templates and settings from a parent theme while allowing selected overrides.
When it helps
Use one for template overrides or custom CSS/PHP not handled by theme settings or a site-specific plugin.
Avoid copying everything
Large numbers of copied templates make future updates harder because fixes may not reach your overrides.
Back up and test
Keep versioned changes and test after parent-theme updates.
Further reading
This guide is educational. Check current documentation before making technical or purchasing decisions.