When the character responds to the archdruid's call for aid, they are first asked to bring a number of samples of the soil of Un'Goro back to an assistant. The archdruid then explains their program for researching Morrowgrain, which they know little about. By growing Morrowgrain for the Cenarion Circle, the character is given a bag of various goodies - food, water, some potions and elixirs. And the need of the circle for more morrowgrain for research is endless - provide them more, and they will give more of these goody bags.
Not all of the Circle trust Staghelm, though. Quintis Jonespyre at Feathermoon Stronghold has also expressed interest in doing independent Morrowgrain research, although only members of the Alliance are able to talk to him without starting a fight.
After the Lich King's defeat, Staghelm's purpose for the Morrowgrain was revealed: he was using it on Malfurion Stormrage to trap him in the Emerald Nightmare and, eventually, to kill him. The healing power of Elune, as channeled through the priestesses caring for Malfurion, kept him alive until he was able to escape the Nightmare and expel the poison from his body.