Adding a Friend lets gives you 5 more points to spend on your stats.
Removing a Friend removes 5 points from your highest stat for the first friend removed per day, 6 for the others.
You can have as many friends as (1/2 X Player Level + 1), with a minimum of 10 friends and maximum of 50 friends up to player level 500. Then the limit is gradually raised up to 60 at player level 700.
You can call your friends in a battle against a raid boss or quest boss in order to get help to defeat them.