It's really interesting to figure out all of the legendary pokemon's purposes and how they were created and the roles. I solely believe pokemon came first, as someone stated, "fossils". It's odd of how we learned FIRST, was Mew, creator of all. Then 3 generations later, Arceus. I felt completely mind-blasted when I started connecting of all of these.
Mew - Creator of all (correct me I'm wrong, it's what I solely remember without accurate info) 3 legendary birds, didnt have much a purpose but be know for their types (except for Moltres, who had a purpose for its fire to light the pokemon league). Ho-Oh, rainbow bird that's keeps the peace but related to rainbows, Lugia, a legendary pokemon that lives among the whirlpool islands. 3 dogs, didnt have much to them either besides Suicine ability to calm down pokemon or settle nature down with its howl. Entei was featured in a film. Also celebi was a pokemon for time traveling (I find it cool but not much of a purpose)
Groudon - of earth, (volcanos, lava, magma and etc) Kyogre - of water, (bodies of water and the depths of the ocean, etc) Rayquaza - of the sky and "ozone" layers as someone had stated. Those regi pokemon didn't have much purpose unless its was Regigas, a protector or guardian of a certain town, or guardian for safety when stopping the major avalanche in the movie.. Jirachi was a wish making pokemon. While Deoxys is an "Alien" pokemon. Dialga - of time (I believe) and Palkia (space), Giratina - underworld/otherworld, Arceus - another creator or partners of creating everything ith mew somehow. (Enlighten me please) then the 3 pokemon that were the senses, mind, knowledge, willpower? I'm not entirely sure.
So far, history of legendary pokemon purposes are coming to an understand, if any can understand me. Then the story for Zekrom and Reshiram, both were split parts of Kyurem. As Kyurem is a vessel or a empty-being pokemon, who once had Reshiram and Zekrom together as one being but I don't know the purpose of that. The 3 swords of justice, fighting and other types of pokemon, were guardians to keep the peace between humans and pokemon. And all that so far, I cannot tell how to be perfectly clear with pokemon.
(I apologize for my horrible grammar and sentence structure, if anyone cares)