flex-wrap:balance allows developers to distribute content between flex-lines so that it appears more balanced (similar to text-wrap:balance).
Without balancing it is trivial to create flex-lines which appear "unablanced" (lots of whitespace in a particular line). This often makes developers avoid wrapping flexboxes, instead allowing content to overflow. See explainer.
Explainers: https://github.com/bfgeek/flex-wrap-balance