New players running Missions should check out the page Nation Missions with Masterkey for some notable uses of Masterkey to hasten their experience.
At one point, it was required to complete Nation Rank Mission 2-3 in order to use Masterkey. This is no longer so.
Sayeth former developer KnowOne:
It used to. Then it was just set to gm 0 for unknown reason. Then changed back and was some sort of uproar so reverted back to 0. That's last I know