Pretty simple concept; there are global variables (applies to everyone) and player variables (applies to just the character).
Account Variables would apply to the account and all of the account holders characters.
Ways you could use this:
If an account has a character that has beaten the tutorial/intro in the past, then give the player an option to skip a tutorial/intro.
Unlocking something after you have 2 (or more, its up to you) characters that have completed a quest or an end-game boss.