We use this form of words to show we are talking about a future action that is connected to another future action.
This is without the ...ing form which we use to show that there is a point of time involved.
I will have painted the room when he rings. (Both actions are in the future (Painting and the phone call) they are connected and the painting is before the call.
I will have talked to him before he goes. (The talk and the going are in the future and the talk happens before the going.
Why will the bus service have been cancelled before I need it? (The bus service is running now but it will be cancelled in the future and this will be bfore the unlucky person needs it.)