The behind the scenes reason why Susan got a job at a video rental store is because, when I was younger, I always imagined it would be a job I would relatively enjoy.
I know now that I wouldn't, but that's mostly to do with standing in one place, which is not unique to video rental. I would probably really like the environment if I could move around more and have a seat at times.
Anyway, less about me, more about Susan (though there's a LOT of overlap between us in this storyline).
I figured Susan working at the video store would either be a bid for independence and self-reliance on younger Susan's part, or something her mother insisted she do.
Given what all we've learned about Susan over the years, I don't imagine young Susan would have done the former. She expressed herself in other ways, and she wouldn't have been eager to interact with customers. If she did do something as a bid for independence and self-reliance, it would have been something else.
So it was Susan's mother who, at some point, wanted to make sure Susan knew things about hard work and dollar valuing, and perhaps without enough concern for how busy this would make Susan's schedule, insisted Susan get a part time job.
I mean, I can't argue with the intent, but there are only so many hours in the day, and school's already demanding.