For the normal reason of detaining enemy prisoners.
At first, it suited both sides to exchange prisoners, to maintain civilian morale.
But one of Grant's first acts on becoming General-in-Chief in March 1864 was to end the exchange system. He knew that the Confederates were running out of recruits and could not replace their battle-losses. Despite suffering appalling casualities on his own side, he calculated that the enemy would suffer worse, and would have to surrender.
Meantime the prison camps became shockingly over-crowded, and the one holding Union troops at Andersonville, Georgia, was notorious for brutality and starvation.
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The Confederate POW camp at Andersonville in Georgia was the most notorious.
Yes, in Sioux Falls and Yankton. Edit: The Sioux Falls and Yankton camps were branch POW camps of the main POW camp in Algona Iowa. The POW camp in Algona had a total of 34 branch camps in IA, MN, SD and ND. There was also main POW camp in Igloo SD that administered six branch POW camps in western SD, including POW camps at Ft Meade (near Sturgis) and Belle Fourche. POW's did much of the stone work on the grounds of the Ft. Meade VA hospital and they were used for farm labor in the wheat and beet fields in the area.
Begin your research with websites concerning WW2 POW camps. Go to www.mansell.com Extensive lists and rosters for Japanese POW Camps
POW camps for Communist (NVA/VC) Prisoners of War were operated by the South Vietnamese Government: See website: Prisoner-of-war Camps.
Approximately twelve percent death rate for Confederate in Union POW camps. The death rates of Union soldiers was slightly higher in Confederate POW camps.