There are advantages when you become friends with other players. You can earn more pirate points whenever you choose a friend's Pirate Leader as a reinforcement in battles. Moreover, you can share certain quests with each other.
Your friends' Pirate Leaders will be prioritized and shown first in the list of available reinforcements. Keep in mind that you can only use help from a certain friend once a day.
Getting More Friends
After a battle, you can send a friend request to the player who helped you. After a PvP battle, you can send a friend request to the player you fought. You can also add friends by their ID, go to "Sub Menu" tap on "Friends" then on "Friend Search". You will be friends with that player once they have accepted your request.
You can only have a limited number of friends but that limit will increases as you level up.
Friend ID and Acknowledgement Friend ID can be viewed via the "Friend Search" tab in the "Friends" window. You can let your friends know your ID so they can send you a friend request. You can accept pending friend requests by going to the "Friends" menu under the "Sub Menu" and then tapping on "Awaiting Approval".