I wrote down some ramblings about this subject a while back after first reading about it through @mrmrs on Twitter. The idea of single function CSS classes was weird and seemed unnatural at first, but I was very intrigued. I considered this idea around the same time I was realizing that, at work, I basically keep creating variations of the same designs over and over. Thus bloating our CSS file and wasting time developing components that were pretty much previously completed. Then a lightbulb went off. 💡
We were in the middle of a project at work and I decided to try something. I'd create a bunch of "helper" classes that would perform single functions on an HTML element. For instance, .text-center would align the text in the center of the element. .container-800 would create a container that was 800px wide. I created about 20 or 30 of these classes and tried to come up with rules that I routinely use when creating BEM or SMACSS style components.
The result? Incredibly fast iteration! I could piece together these classes in HTML to make interfaces in our CMS so quickly that I couldn't deny there is some real value in this.
Another result: people who work on my team can no go forth and build awesome interfaces without necessarily needing to know about SCSS, preprocessors, etc. It lowers the barrier for entry and helps groups work together faster.
It makes designer and developers speak the same language. "I think there should be a little more padding on this container" can now be remedied by the designer or the developer by modifying the padding class on the element. If the designer doesn't know CSS or any of the tooling the developer uses, it is still possible for the designer to change the interface to their liking. This is so powerful.
If you'd like to check out a library that has created tons of these "helper" classes already, look into Tachyons. It's a wonderful concept and library.