1. After purged the CSS file is really, really small.
2. Hopping into all of my projects and not having to remember new class names.
3. Not having to come up with new class names.
4. Not worrying about making a change in a class and having unintended consequences.
5. Not skipping back and forth between HTML or CSS files.
That said, if you don't dig it, there are plenty of other great frameworks, naming conventions and approaches that people use. Or start from scratch and roll your own.
For frameworks I flirted with InuitCSS for a bit. It's pretty nice and after digging through the code I realized it's not as complex as it first appears.
I also like Andy Bell's approach with Cube CSS.