When trials were being conducted in America initially, judges were able to assign sentences to crimes as they saw fit since there were no guidelines for them to follow. This of course became a problem because if you were tried for a crime in one state you might end up with a very different sentence than you would in another state. This made the system unfair.
Since this was not a fair way to have a federal legal system, the United States had to change the way that they did things. They ended up offering up federal sentencing guidelines which would give judges a range of sentences that would fit certain crimes. These guidelines suggest a range of sentences based on the severity of a crime that is committed.