In my opinion, the best reason to use it would be if you don't have experience or knowledge with any other CSS frameworks. I think getting the experience in understanding how a framework or library works is more valuable than the syntax or semantics of the framework itself. This way, you are prepared to switch to another framework if warranted (or create your own).

Still used, but I'd rather go with pure flexbox + lightweight UI libs (for buttons, etc).

Once you learn it, you can also use it to layout native mobile apps with react-native, or hybrid with ionic...

https://facebook.github.io/react-native/docs/flexbox.html

For non-front-end developers, it's an important and useful tool, because it lets us build things without having to know CSS. But none of the experienced front-end developers I've ever worked with will use it, and most of them wince slightly when its name is mentioned.