Good for a vast assortment of things.
Say you wanted to make a mission where the client had to kill five Ogres. The events would go:
0 [0] OnActivate>Client>AddMission [Ogre Hunter] [5] [1, 2]
1 [0] OnMissionComplete>Client>CenterPrint [You did it!]
2 [0] OnMissionComplete>Player>AddItem [Better Sword]
The first value for the AddMission event is the name of the mission, the second value is the number of MissionIncrement events you have to trigger, and the third is which MissionComplete events to do.
Next you event the Ogres.
0 [0]OnBotKilled>Client>MissionIncrement [Ogre Hunter] [1]
The value dictates how many ticks you are closer to completing the mission. In this case, when you reach five ticks, you get an encouraging CenterPrint and a better sword. It'll usually be set to one.
What do you think?