Why does the prairie need to be preserved?
When settlers moved out west, they converted the prairie to farmland and rangeland. This made it a very different landscape than it was originally. Native grasses were replaced with exotic species to feed cattle and produce crops. Concurrently came the reduction or elimination of native wildlife species, including American bison, grizzly bears, black bears, wolves, prairie dogs and so on.
The alteration of this landscape in favor of cattle and agriculture affected all wildlife. Pronghorns, for example, face an obstacle in the fencing. To forage for food, they move from one ranch to the next. Manmade ponds that were created for the cattle, too, disrupt the grazing patterns and the distribution of native grasses.
The bison, the prairies, the mesas—they are America. The land APR is restoring is where the iconic Lewis and Clark expedition first saw vast herds of wildlife. You can read their journal and stand in the exact place they were standing when they wrote it. In it, they talk about the wildlife, the vistas and especially the grasslands.