In poetry and fiction, the main reason for using vivid words is to

Be able to paint a clear picture for their reader.

The answer is, "to create a mental image".

The idea of Manifest Destiny was music to the ears of frontier settlers, but meanwhile, the sound of Manifest destiny to Indians was distress and chaos, because they knew that they would have to be kicked off of their land without any say in it.  During a situation like that, Indians were basically helpless. Hope that helps.