Question

What might have been a reason that Allied soldiers forced German citizens to visit the Buchenwald death camp?

Answer 1

Probably to be witnesses of the horrors perpetrated in the camp and not be able to deny it or refuse to believe in it.

Explanation:

One of the big problems about the scale and human dimension of the horror connected with concentration camps was credibility.
Nobody could imagine at the time a crime of that enormity, savagery and viciousness so simply thought of it as an invention an exaggeration.

Probably the American soldiers and Gen. Patton in particular decided to deny the possibility of cancel or forget the enormity of these crimes by having the same people, living few kilometers away, to see and touch with their hands the reality.

[Local residents forced to confront corpses and other evidence at newly liberated Buchenwald Concentration Camp]

These people were going to become the real witnesses of the horrors perpetrated by their own fellow countrymen.
Nobody could say anymore that was propaganda or an invention of the victorious enemy troops!