In BG2, each companion character had their own goals and quests, and I think the BG3 Origin stories are similar to that. Edwin wanted the Nether Scroll, Anomen wanted to be a Knight of the Radiant Heart, Jaheira had to get rid of a curse, etc. The system in BG3 simply allows you to play as that companion instead.
And while you may be locked to a race and class (like the companions in BG2), 5e is great because there are subclasses that most classes will be able to choose at level 3 (level 2 for Wizard). So, if you're Gale, I'm sure you will be able to pick one of the specific Wizard subclasses to customize your playthrough.
