My Name is Earl started out as a big hit, but throughout its four seasons, ratings started to drop. The premise of the show focused on Earl crossing off a list of people he'd wronged after doing something good for them. Due to the show's cancellation, he never finished the list. An extra season would help give the story a proper ending, and give the show's creator a chance to give us a real finale, not the one he teased during a Reddit AMA:

You’ve got a show about a guy with a list so not seeing him finish it is a bummer. But the truth is, he wasn’t ever going to finish the list. The basic idea of the ending was that while he was stuck on a really hard list item he was going to start to get frustrated that he was never going to finish it. Then he runs into someone who had a list of their own and Earl was on it. They needed to make up for something bad they had done to Earl. He asks them where they got the idea of making a list and they tell him that someone came to them with a list and that person got the idea from someone else. Earl eventually realizes that his list started a chain reaction of people with list and that he’s finally put more good into the world than bad. So at that point he was going to tear up his list and go live his life. Walk into the sunset a free man. With good karma.