CSS on its own can be fun, but stylesheets are getting larger, more complex, and harder to maintain.
This is where a preprocessor can help.
Basic stuff is really simple.
Some of the more complex stuff is not that difficult too.
As to the rest of really complex stuff: unless you have dedicated your career to be a front end developer, or event more, a front end designer, then most likely you cannot really grasp how you are going to design your stylesheets using advanced SASS capacity from the get-go.
If you are like me, a non-highly-paid-expert, you are going to use preprocessor in its basic capacity with some semi-advanced functionality. And when your stylesheets are going to grow huge, you will start optimizing them using more advanced SASS functions.
Some useful links: