Sometimes one would want to limit the growth of a specific minor population in their empire. For example, if all of your planets are terraformed to fertile, Haroshems bonus food on Sterile isn't as attractive. Chang gang (and similar abilities such as the Virtual Dustylizer, that can be found from a curiosity and converts the population unit to dust) seems like it would help with this, but not as much as it could. 


Each pop type present in a system has a "chance to increment" that (unless it is a Riftborn or similar species that cannot reproduce) increases by 50 per turn normally, or by 100 per turn when the species is boosted. When you get enough food, the species with the highest value for that number gets a new population unit. After this, the chance to increment resets to 0, so the next population unit might be a different species.


But if you use chang gang, you get 300 manpower and the population does not grow. However, the next turn that population will still have a high chance to increment and will likely gain another unit the next time the food stores are filled.


Chang gang (and Virtual Dustylizer, etc.) could be more useful if it reset the chance to increment to 0, putting it back at the bottom of the list for chance to grow. It is as if you are letting members of that species have some babies and then immediately conscript the parents into your navy, so your number of Haroshem does not increase. This would make it more useful for population micro. So if you don't want another Haroshem in a system, you can chang gang it and then give another population type the chance to grow.