Because on the online mode you go in a team of 4 players, each with different weapons (if u have the same weapons as someone else it is entirely coincidental), and you play as Inklings, squids that evolved to be humanoid and live on land, along with all other sea creatures, millions of years after humans whent extinct, and they made a sport called "Turf war" that you have to get as much turf as your color of Ink to win, there is also "Clam Blitz", "Tower Control", and "Rainmaker". They all require teamwork to win, therefore Splatoon teaches teamwork