Why should i add the butter after i knead the main ingredients when making a bread?
In a yeast-risen bread, like the banana bread video you linked to, it's important that the liquids in the recipe are allowed to properly hydrate the flour particles. This is so that the gluten proteins present in wheat flour can start to develop the gluten network needed to contain the gases produced by the yeast. If you add fats such as oil or butter along with the other wet ingredients, they can coat the flour particles and prevent them from getting the moisture they need.