You know, I like Mo Xiao Nan. I have to admire her bravery for constantly trying to get closer to Sun Jing, even when she doesn't seem interested. There is a point at which she has to accept Sun Jing has no interest in her, which will likely be the end of this arc, but, to say she's only there as a love rival I think is unfair to her. This update makes it clear too, what the depths of her feelings are, and I always like characters who I can relate to in some aspect. The bullying scene shows an element of bravery and naivety that felt very real. Not thinking things through completely, expecting somebody who's too scared to even stand up for themselves stand up for her, not blaming them or even the bullies for the situation she's in, but claiming responsibility for her actions. I like that. If it was Sun Jing, she would have charged in without hesitation and somehow or another forced the bullies to stop, because that's how bullheaded and charismatic she is. This is a different scenario, of somebody who aims to do something but isn't lucky? strong? enough to carry it through. And here is Sun Jing showing up at just the right moment, displaying what Mo Xiao Nan lacks and desires.
Story-wise, if Xiao Nan needs to have a "reason" to be here, then yes I do think it's for Qiu Tong to show she's serious. But this arc and Xiao Nan's actions have been built up over time from the very beginning, her first appearance is in the second update, and this is not a random, half-assed melodrama act.
Not that it matters, but in the Weibo comments for this update, roughly half of the most upvoted comments are people appreciating Xiao Nan as a good person/somebody with a story/understandable/with compelling feelings. (The other half are joking that Qiu Tong has been squatting by the gate since last year but I digress.) So for the original reading population at least, there's no arguing about Mo Xiao Nan's place.
A normal way to do this is have them confess and the main character turn them down. But instead we get a stupid flashback making the main character out to be their knight in shinning armor (and if I know drama the girl has way too much wrapped up in getting Sun Jing's approval) leading probably a half a year of updates dealing with drama since this is building up for a few chapters despite the girl being a side character.