Zoey needs a backstory because the relevance is already there.
Relevance:
She is MC's ex-girlfriend. The MC didn't know when Zoey left if the pain in his chest was because of love or because of the loss of his best friend. The MC himself said to Maya in EP2 that he doesn't really know what love is. In EP9, maybe already in Interlude, DPC brings Zoey back together with the MC and he has a LI (or not). Thus, the MC can explore his feelings and gain the experience of what love feels like. The relevance to Zoey return is there. The MC has a relevant way to compare with the feelings he felt for Zoey and the feelings he has with his currently LI.
Backstory:
The backstory is important because we need or should learn the reason why Zoey left. Yes she wanted to learn to surf, that's what we know. But one little thing bothers me. Zoey wanted the MC to come with her. To me, that makes it seem like Zoey was running away from something. But from what?
I'm thinking that Zoey found out about the whole family mudslinging from the MC and that's why she went to San Diego. She wants to protect her MC from it, so she asked him to come with her.