Why Joomla Override?

When i was working at Diligent Tech, i used to hack extensions files directly, might be my seniors did not about know the term 'Override' or they haven't informed me. When i joined Tekdi Web Solution, I come to know disadvantages of hacking core files. Override is the great solution for hacking core files. So I'm just posting about why override instead of hacking files.
When you want to upgrade Joomla component or module installed on your site. You uninstall it & install that component or module with latest version or you can directly install the upgraded version. This means that the changes you made in core files of the component or module will be lost if you don't have a back-up. It's also more difficult to keep track of the files that you have customized. With the override functionality you can place your customized files in a template folder which is of course outside of the component or module to keep all your customization in one place and safe from upgrade losses. So that your colleagues will get in which files you have made change.
But one more thing you should check the file differences when there is an upgrade, because there might be bug fixes in your modified files, but now you can easily keep track of which files to check. Instead i write about how to override joomla core files please check this link http://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core.