When you have a mutable struct in which you want to have a field ref readonly (a value you get by reference that cannot be modified by the caller) that does not mutate the struct.

You can find quite a good explanation here: "Readonly Ref vs Ref Readonly in C# Struct"

Now, as to "real-life" use cases... that's probably for performance-sensitive code, where copying the struct is too expensive.

Also, it's not really "new", it's possible to do this since C# 7.2: Microsoft Docs | Readonly references