WinRT can be used to create COM (DLL files) with C#/VB/C++.  COM is pretty easy to do in C#/VB, so I am going to skip that story.  WinRT with C++ is MUCH, MUCH, Simpler then making COM in C++.

So there is a reason: WinRT is easier to build then COM.

Now you are  building a 30 Day To Launch Game and you find out you need to speed up a calculation.  One of your friends writes in C++ and you need to connect with their work.  WinRT is the answer.

Another Reason: Connects with JavaScript through the Application Binary Interface (ABI) boundary to C++

girlie_peace