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.
ASP.NET Core (web framework for C#) is one of the best available, when you need great performance. Also, C# is pretty popular and that means potentially more contributors.