system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 4] Why is the Institute stealing and replacing people?

I understand their endgoal. It's similar to the Master in Fallout 1 in fact. The Synth is a net upgrade on the basic human. Immune to radiation, no need to sleep, physically stronger and more durable, etc... They are an existence capable of surviving in the wastelands and potentially rebuilding in a way that humanity has struggled to do over the past 200 years.

But why steal and replace people? What possible motive could there be for willingly making yourself the villain of the Commonwealth?
gpt
You can argue about the practicality of the plan, but it makes sense. 

They want control of the surrounding Commonwealth area. But they're not very popular; certainly not enough to walk into Diamond City and openly run for Mayor. If they snatch up people of influence and replace them with perfect duplicates, then they get to discretely guide every settlement according to their own agenda. Everyone dances to the Institute's tune, without even realizing they're doing it. 

Some of them might also be loose ends getting too close to the Institute, wrapped up to preserve secrets.