The Legendary Pokemon are the myths of the Pokemon universe. In fact, plenty of legendaries are based off of mythological beings. ike in real life how myths are usually used to explain something, the legendaries account for things in the Pokemon Universe. Arceus came first and created Dialga, Palkia, and Giratina to create the universe, and then Arceus created the Lake Trio to control the Dragons. Mew comes in like a little Stem Cell and has all the coding for all the Pokemon. These Pokemon then create other Pokemon to do things to help create the world. All this creates a background that adds an interesting story to the games.