The variable is_preview lets you know if you’re in the preview mode for Gutenberg or on the front end. It’s useful when you want to display a custom message to users when they edit the block on the back end. See my screenshot https://imgur.com/s295oqkhttps://imgur.com/r9025Zv
What I hear you saying is that Meta Box Builder does not yet have a way to set the block setting preview, so it is not possible to generate the small preview with Builder. And if that is important enough, we should code the block in MB Blocks instead.