In Ch. 13 the Giver explains to Jonas when and how he advises the Council of Elders.
"Rarely. Only when they are faced with something they have not experienced before. Then they call upon me to use the memories and advise them" (Ch.13).
Because the Council of Elders and everyone else in the community has no collective memory of their community's or world's past, they do not have the advantage of learning from past mistakes and using that wisdom to inform future decisions; therefore, they call upon the Giver to provide them with the context they might need. He can step in and share his advice based on that knowledge, without having to expose them to all of that knowledge. He does this when the plane flies into their air space and they want to shoot it down. He tells them that being hostile might lead to unintended consequences, including war and hunger.
He also travels to neighboring communities as well, when necessary, to advise them.