Sometimes I feel flexbox was created in order to get back some cool features of table-based layouts without using tables. Technology tends to run in circles.
But table layouts just happened to be very similar to grid layouts, which was what web developers wanted in the first place but CSS as a language didn't explicitly support.
Flexbox was built for dynamic grid layouts, and is a huge step above display:table, creating rules that are explicitly meant to do what you could somewhat hack with table rules.
CSS has this strange acceptance of hacks being a part of the language when it shouldn't be. The fact that you vertically center things with pos: absolute, top: -50%, and it is considered normal is abnormal for a language. Good language design is not to find hacks that allow you to accomplish your goal (robust layouting) - its to fix the language to give you a proper toolkit to accomplish your goal.