This grammar pattern is used to show that the action is carried out right after the other action in completed. Those two actions are usually not happened yet.
● 그 사람을 보는 대로 나한테 알려 줘. (Let me know as soon as you see him.)
● 여행에서 돌아오는 대로 전화할게. (I’ll call you right after I came back from my travel.)