What made the McCormick Reaper important was the fact that it allowed farmers to cultivate a larger amount of grain. This was because it allowed them to harvest the grain much more quickly than had been possible.
Before the invention of the reaper, grain had to be harvested by people who cut the grain by hand. They used long, sickle-like implements called scythes. Any given person could not cut a very large amount of grain in one day. This is a problem because grain needs to be harvested very soon after it ripens. Otherwise, it can start to rot. This means that the amount of land a farmer could cultivate was limited by his ability to harvest all of the grain produced before it went bad.
The reaper greatly increased the speed with which a person could harvest grain. The reaper cut the grain mechanically (while being pulled by a horse) and could do it much faster than a person with a scythe. This revolutionized farming by vastly increasing the amount of grain that one farmer could grow.