Every second counts. Your web page will load faster if the css file is smaller. The output is the same visually, but the page loads quicker.
Minified CSS is not easily read by humans. We like line returns and comments.
You can save a version of your .css file and name it something like 'styles-unmin.css' to represent the unminfied version. You can then edit this version, come back to this page and generate new minified css to apply in your 'styles.css' file. Then just reference the smaller styles.css file from your webpage, and keep the unminified version available for future edits.