I don't need it. I want it, because C# is my favorite programming language. Isn't that what freedom is about?
This means that when I'm writing a website or a Docker microservice, I want to use C#. And since the best OS to run those is Linux, there are no proprietary MS products in sight.
When I'm writing a cross-platform desktop application, I want to use C# too, but I can't, so I'll use another language.
Why do you need this on Linux? Only to integrate with MS proprietary products, that's why.