I think the central issue here is: javascript has only recently turned the corner of utility in most environments. It used to be the toy scripting language that you used in order to tie form fields together. I used javascript back in the day to send events to a java applet that handled rendering molecule pdb files (jmol).
Nowadays, and quite suddenly, javascript is at the center of everything. Java applets? Flash? Everything's imploded. On top of that, we're revolutionizing the language (block-lexical scoping, classical OO syntactical sugar, standardizing import/export, promises, generators, async/await, optional type annotations) in a way that improves productivity and the reach of the language.
There's a lot of churn in tooling and tools, because things are new. I've been doing software since before the STL and generics in c++, and I can say that I've seen this kind of churn before. It happens with lots of languages when they turn the corner in real exponential growth. I'd say it used to be the case that there were far fewer programmers in general, and they were a far more homogeneous lot than they are now (we still have a long way to go on that front, of course), so standardizing around autoconf and make was easier than standardizing around (say) ant vs maven or grunt vs gulp.
To the original issue - I've been doing a lot of flexbox work lately, and one of the more annoying issues is dealing specifically with grids. If you just use flex-grow like a good flexboxer, stuff doesn't align well in columnar format on your page. Flexbox is great for some things, but handy macros for making a grid out of flex boxes are pretty useful. Useful enough that I ended up writing pretty much the same thing myself anyway, and useful enough that it makes sense to hoist some of that code out into a standalone library. That way, you can encapsulate fixes for browser quirks (shakes fist safariiiiiii!), and have the option to pull the whole thing into new projects or leave it out if you don't need it.
People can complain about making a spiffy project page for a small library, but hey. People complain about a lot of things. Those kids, for example, on my lawn. I installed a motion-controlled sprinkler to keep them off, but those wacky kids are still on my lawn.