I'm not familiar enough (or at all) with C#, but AFAICT, it could make an instance more stable, as firefish and misskey have struggled with handling a decent amount of users and C# could be a faster system for the server.
Also, a re-write sometimes is a good thing. And, developers have different preferences for languages, so having a C# project around enables C# devs to more easily contribute to the fedi.
The biggest advantage to C# is that it is a strongly typed language, which allows the compiler to do a much better job of verifying your code. ("Type" in this context refers to data types.) Javascript is a weakly typed language, so the compiler will accept a lot of code that C# wouldn't. So your users end up discovering bugs that a C# compiler would have discovered during compilation, so you'd fix them before you deploy to users.
Typescript is a big improvement over Javascript, but still nowhere near as good, mostly because it needs to maintain backward compatibility with Javascript.
Weak typing is convenient for small/simple projects and for people who are just getting started with programming - both of which are what Javascript was for. But the bigger the project, the more useful it is to have a strongly typed language, and a compiler that will point out type mismatches before your users get a chance to find them.
