Sharplab

public class C {
    public int A(int i) {
        i += 2;
        i += 4;
        return i;
    }
    public int B(int i) {
        var j = i;
        j += 2;
        j += 4;
        return j;
    }
    public int ConsumeA(int i) => A(i);
    public int ConsumeB(int i) => B(i);
}

According to @SingleAccretion, this is because Roslyn prefers to use the IL stack for locals instead of parameters.
So an optimization would make sense for this.

I would argue that there is a real-world case, as I have seen code where no new variable was created but the parameter was modified directly.