I mean Dr Pink Cake put that choice in there for a reason
I'd say it's there to move along Zoey's plot and add to the internal strife among the DIKs. It's something for Jacob to feel bad about.
Zoey is the dark horse of this game. No matter which girl you choose Zoey the ex is a big challenger. No matter how much of a manwhore you were in the first two seasons the main girl you chose at episode 8 won. She was better than all the others. She might be pissed that there was so much competition but she won. But she hasn't won against Zoey because Zoey wasn't there when the choice was made.
All the girls have a different fantasy at their core. Note that I see these as merely the starting points of the creation of the characters. There is a lot more to them now and they might not have all the typical ingredients, but these were the starting points.
Josy - Girl next door. Well same workplace in her case, but she is the pretty girl you have a crush on and see on a daily basis but aren't even properly acquainted with. When the ice gets broken it can go to a lot of places, but a big part of the set up is that the ice has to be there for a while.
Maya - Lesbian discovers she is bisexual. You are the first guy she has ever found attractive. It's way more than just being a girl's first fuck. She is very confused about this new part of herself, but she likes it. She really, REALLY likes it.
Sage - Party girl's first real love. She has had a lot of fun, but it has never been emotionally serious. She regrets her past relationships as something she wishes she hadn't done in the first place rather than as something that didn't work out. You are the first guy she has actually fallen in love with.
Jill - Sheltered rich virgin. Overprotective parents keep their daughter "safe". Her home isn't her safe castle it's her prison. You are the hero who rescues her. It can get very fairytale.
Bella - Rekindling the older woman's passion. Her heart's been broken and she's given up on love. But then she meets you and maybe it's not too late for her after all. If only it weren't so wrong for you to be together. Oh, who are we kidding, it's a massive turn on. But can there be an actual future with someone so much younger?
Zoey - The ex that wants you back.
I'm not going to list the side girls. This took long enough as it is.