Friends benefit you in a couple of different ways in Speedy Ninja. When you die in game, you’re given the option of using your friends’ characters to continue your run for free. That means that you get to take control of a Ninja with the exact same stats, weapon, mount, and level as your friend’s character.
Also, friends have the option to provide you additional Energy that you can use to feed your Mount and complete extra runs. This gives you greater chances to unlock sweet in game rewards.