To raise our chickens outdoors on pasture while keeping them safe from predators, we provide them with mobile shelters that provide shade, protection, and feed and water. The shelter design has removable wheels so we can wheel them to fresh grass throughout the season and the chickens don't spend too long in one place.
The bottom of each of these "chicken tractors" must be flat. There is no floor, which means the chicken manure fertilizes the pasture as they go. So the edge of the shelter must be flush to the ground so predators can't reach underneath it while the chickens are inside. Putting a half-lap joint into each bottom corner helps ensure it stays flat and there are no gaps. And even though the other corners could probably be held together by just a few screws, putting in joints will make the structure stronger. Also, it just looks better!