When you look at how admins get their job done, it starts in the interactive shell and then when the do things frequently, they put it into a simple script and when then need it to be more formal, they invest in making it production quality.
That is the flow that PowerShell was designed to support.
Jeffrey Snover [MSFT]
It's worth noting that all code in .NET is enclosed within a type (at a fundamental level .NET code is either contained within a value type or reference type, but knowing those names isn't important when you're starting out), so this doesn't limit you to creating/importing data types, you can use it to write inline C# functions too.
It's also interesting to note that the .NET Framework comes with its own .NET compiler, so you don't need to install any dev tools on Windows to code in C#, it's possible to do so with the tools that are available out of the box.
If you want to try out referencing C# code using Add-Type, the following article is a good introduction:
https://blogs.technet.microsoft.com/stefan_gossner/2010/05/0...
It's such a shame powershell didn't come 2 years later, I feel like they'd have had no choice but to use C# instead of inventing a new language.