Confit was created by the French as a way to preserve meat before fridges came along. They would marinate the legs in course rock salt before poaching in the fat in order to draw out the moisture. The longer you marinate the legs in the salt the longer you would be able to store it. I omit this stage as I find the meat really salty but it does mean you need to remove the juices from the used fat before storing. The slow cooking tenderizes the cuts of meat that have more connective tissue ie the legs.
Of course this method can be applied to game birds too. The smaller the leg the less time you will need to marinate and subsequently poach in the oil.