America had become divided over the morality of slavery, so much so that a whole war was fought over the issue. The only way for America to survive was through the emancipation of slavery.
Cows could roam freely without having tight spaces and fences to bother them. Since they were branded, there was no fear that someone could steal them and resell them because it would be easy to recognize the brand of the owner. Since the west was basically a giant field without people, completely empty, cows could do whatever they wanted.
Any third world country, are there certain choices?
To supply more soldiers for his army