If you ever worked with something like SCSS, LESS or any other CSS preprocessor, you don't want to go back to plain old CSS. Working with SCSS is just a joy, especially when you start realizing that copying styled elements is much easier, that variables are making your life much more fun when the customer wants to change the colours once again and much more.
That's why we don't want to go back to CSS too, but like lots of people, we have to work with old code from the days before CSS preprocessors and they just aren't as much fun, especially if you know how good things can be. This is where CSS2SCSS.COM jumps in. It won't create perfect SCSS code, but it gives you a good start by created a properly nested SCSS file based on your existing CSS file!
Want to help? The library powering this site is open-source, checkout its github site if you want to contribute: https://github.com/ortic/css2scss