There's different uses for the do_action hook in WordPress.
One way to use do_action is for creating new hooks which i have already written about.
This enables you to add a custom hook in your parent themes template.
You can then use the custom hook in an add_action function in your child theme to hook in new content, a widget or a template tag for a slider as one example.
You can also add a conditional tag after the function for more control over which pages your function executes.
The custom hook can be used many times so you don't need to keep modifying your parent themes files.