Cannot use mutating member on immutable value: function call returns immutable value: why cannot call mutating func directly on return value?

Using Swift
3

What you say make sense if the mutated value is used afterward. But for an JSONEncoder:

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .custom { date, encoder in
    var container = encoder.singleValueContainer()
    try container.encode(date.formatted(.iso8601)) // what happen here?
    // nothing more
    // the code read like none sense, illogical  :( 
}

My guess is inside container it's "wired" so calling .encode(...) cause the change up in the encoder. So why would a "caller" need to use an intermediate var? Seems like unnecessary leaking of implementation details.