As I understand, the idea of immutability and usage of packages like Setfield.jl to change struct fields is a safety feature for concurrency. When putting it in

As I understand, the idea of immutability and usage of packages like Setfield.jl to change struct fields is a safety feature for concurrency. When putting it in as an argument to a function, you can be sure that it would not be changed during execution introducing races without needing to acquire a lock. Also, state machines become easier to reason about when immutables are used.