Flex is good for creating flows of content, like a <div> where all childs has display:inline-block. It supports wrapping when content can't be on a single line.
Exactly. But we came from using Flex as the defacto system to do layouts and more, now with CSS-Grid it does make sense to use flex for what it was meant for.
I guess it depends on your past experience, but for most of the teams I've been involved in, it was basically a leap from floats to grids for page layout, with flexbox being a small scale option before and after.
Grid is good for making grids.
Flex is good for creating flows of content, like a <div> where all childs has display:inline-block. It supports wrapping when content can't be on a single line.