"We write the letter and you write the message."
Translation:Noi scriem scrisoarea și voi scrieți mesajul.
It seems like "iar" is very commonly used. It is prefered here because it emphasises the fact that there are two actions taking place at the same time. I wouldn't want to risk giving a false translation ("iar" really means "and") but I see it as a sort of "while": I'm doing this while you're doing that".
That's what my good Romanian friend tried to explain to me. It's worth what it's worth. You shouldn't try to translate it. It's probably better to notice it's usage in other sentences so as to get a good feel for it.