Why does unreal engine use c#

Unreal Engine is mainly written in c++. But there are some things that are written in c#.

1> So what's the main purpose of using c# (what functionality is written in c#)?

2> And also why was that written in c# and not c++? What are the benefits of using c# in those cases?

I think answering this question will help me (and anyone else who is interested) understand both c++ and c# better.

What what they are each good for. Since there has to be performance issues or functional benefits, for unreal to make this decision.