From Numbers 11:6, I observe that these 70 selected persons were supposedly recognized elders and officers over the people. But in order for them to be empowered or equipped for godly leadership, the Holy Spirit must first come upon them. As good as charisma, or experience(age), or even intellect; these alone CANNOT equip a man for godly leadership or service. Such persons must needs be enabled by the power of the Holy Spirit, as was the case with the 70 elders.
When the Holy Spirit comes upon them, they all began to prophesy; which simply means to speak not of one's self, but by the inspiration of God.
Finally, from Num 11:14, I observed that Moses voiced out that he alone was not capable of administering leadership or overseeing the affairs of God's people. Moses knew that if God Himself would lead all His people(by His Spirit), then the great burden that was upon him, as it were, would be lifted from him. And this would only be achieved when the Holy Spirit comes upon God's people individually.