If you don't have a use case that requires FoundationDB you def. do not have to learn it.
It solves a problem that others have not solved. Which problem is that? And how better does it work than whatever there was?
Though "super reliable distributed database presenting a single logical shard to client code" is a class of system for which I don't think there's anything else even close out there, and I suspect generally if that's something you -need- then you'll already know that.