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You should read the proposal thread as it discusses this to some extent https://github.com/dotnet/runtime/issues/67209. Immutable does not simply mean read-only but it means that we can mutate the underlying data structure, yet each mutation spawns a logically independent object for the programmer. The Immutable library is optimized for this purpose just as a truly functional language must optimize for this. The new frozen libs are optimized for read only access.
It's so niche it's probably only getting merged because asp.net core team found it useful.