Take the following code:

    public static string ReverseIt(string myString)
    {
        char[] foo = myString.ToCharArray();
        Array.Reverse(foo);
        return new string(foo);
    }

I understand that strings are immutable, but what I dont understand is why a new string needs to be called

return new string(foo);

instead of

return foo.ToString();

I have to assume it has something to do with reassembling the CharArray (but thats just a guess).

Whats the difference between the two and how do you know when to return a new string as opposed to returning a System.String that represents the current object?