What is the benefit of making sub theme in drupal? I have created a sub-theme of a theme in Drupal 7. I have done all changes to create this and it is working fine. But I am confused what actually it is for, what are the benefits of making this? 
I know the differences between base theme and sub-theme but I need more information why we create sub-theme.
For instance. You override only files you need to and you have in your sub-theme directory only these files, not all. You can update parent-theme and not overriding your customizations.