The cotton gin was one of the most influential machines to be invented up to that point, at least in the US. Up until that point, cotton was extremely unprofitable, as it was very hard to get all the seeds out. For this reason, most plantations farmed tobacco. However, after the cotton gin's invention by Eli Whitney, cotton became very, very profitable, and so the South started growing it. This was important because slavery was going out of fashion, but after this, it surged in popularity.