A player can be friend with other players by requesting and being approved, and such players are added on friends list. Friends more often appear as a helper than players who the player is not friend with.
What is benefit?
Easy to ask help
Friends appears on the helper list more often than other players. That means you can frequently use a character which you want to use by being friend with a player who uses the character as a team leader.
Increasing friend point gained
By taking a friend as a helper, you can gain more friend points than usual. You can earn friend points when your character is used by friend players.
How to use friend point
You can roll a friend gacha by using friend point.