The title says it all.
Converse with fellow players
Player ID's are randomly generated upon account creation on the Global Side of things unlike the Japanese side (you would create your account name and password there and verify it with a CAPTCHA before being able to optionally utilized two-factor Authentication to get in-game rewards and benefits).
Your name will be something along the lines of "PN" with a string of numbers. You can change this in the Support Menu for free just once on the Global side before you start paying AC to change your name further.