Develop

The reason humanity has been able to steadily advance year after year, is due to having access to the research of predecessors.

If you however took someone from the current time frame and threw them to the past, the actual effect they'd have on things would be minute.

They just don't have enough of the foundational knowledge required to reproduce many things we currently have. Nature also won't give a damm and will try to kill them.

The most common result by far being that the time travelers end up dead.

Even the ones that visit the current time frame don't have it much better. They usually end up captured, interrogated, dissected, deemed useless, and finally terminated.

Leaving them alive is just a bit too much of a risk. That's also the reason why time travel guides always include a list of black zones. These are time frames and areas to avoid at all cost. These lessons were learned using the sacrifice of many menial servants.