I feel that something happens to the mountain city in the future and he wants to migrate them to Glory City to protect them from what ever happens at that time. He did say they were forced to move on from that city after a while in his previous life and they make food out of wood, so it's possible that there's a famine and they had to kick out all of the new comers to their city. If there's a food shortage, then at least getting them to trade with glory city could help that situation.
Also showing the people of Glory City that there are other tribes outside of their walls might get people exploring more, which also benefits Glory City.