> For which purposes if not software development exactly?

Well, if you recognize only 1 category "software development", I can't help you. C was designed for software development as well, but you wouldn't chose it to do web development, I hope? It's a good tool when you need to develop something small and resource efficient. OOP fits good when you don't have much of concurrency, but you manage complex states in memory (which you do with help of objects and inheritance). And functional programming fits well when you need to manage data flow applications.

And truth is in a big complex system you need a decent support of both FP and OOP. Point is that languages like C# decently support only OOP.