NomadPhp provides a way to use php into a css file. This isn't as bad as it sounds. The php file is parsed, then copied to the webroot as a standard css file. Granted LESS/SASS are much better and probably should be used, this was a proof-of-concept addition. All-in-all, it is useful when you need the flexibility of LESS/SASS but don't want to fuss with setting it up. There are some neat features like: color rotation, lighten, darken; but primarily used to define some global hex colors and allows the ability to change them easily across all your css.
Note: The use of XCss is completely optional and you can still use standard php files.