You might think that math doesn’t have a lot to do with writing stylesheets, but you can actually do some amazing things with a little math in CSS. Math (particularly trigonometry) can help you model the real world. You’ll need it if you want to do something complicated with 3D transforms. And it can be a lot of fun if you just want to impress your friends.
The author, Ana Tudor, also talked about this at CSS Day 2014. Unfortunately I could not attend, due to being in Ireland at the moment.