Why do the community Elders seek the help of The Giver?

The Giver is the sole repository of the community's memories. He is effectively the custodian of the community's past. Although the Elders of the community wanted to banish the past forever to fit in with the general policy of Sameness, they realized that, for practical purposes, they would need to draw on specific memories to help them deal with various problems, especially those arising from unfamiliar situations. For everyone to have such memories would be destabilizing to the whole community, so only one individual can be entrusted with them, and that individual is The Giver.
The memories make The Giver wise, which is why the Elders of the community constantly seek his advice. In one such case, The Giver advises the Elders not to increase the population. He doesn't tell them why, as they don't want to hear unpleasant things. But The Giver has delved into the past and seen for himself the starvation and warfare to which an increase in population could lead.