Effectively another case of him wandering into another story and diverting the focus in the process.
I doubt it will actually be in this story if he gets that far, but it's a necessary moment to have.
Probably because it'd be a little awkward to actually do so.
Honestly, she's just a kid appealing mechanic assistant.
Though the weird tail pull kick mechanism is a bit random.
For the sake of making a different comparison, let's look at Jewel.
We've seen fairly little of her over the series, but that is actually justified and we know what her role in the series is. She's meant to be the Tails to Tangle's Sonic(except more original) and exchanges their big bro-lite bro dynamic for one of childhood friends where one suffers in trying to keep the other from hurting themselves, causing a slew of noodle Incidents that is implied to have made her a bit more capable of action than she'd like to admit. Additionally, her natural organization skills and history of minding an annoying thrill seeker made her a decent replacement for Amy in directing the Restoration efforts(which was probably done as a response to complaints), giving her a proper role outside of her connection to Tangle and likely opening up opportunities for her to actually get involved in the adventure again. All that without either showing up much or getting to show her alluded gameplay abilities because this is a spinoff series that puts the dozen or so game heroes first, not to mention that she is a thing by proxy of Tangle's popularity except not really and the latter is nowhere near as prominent as they would have you believe.
With Belle, she can seem like a similar case at a glance, but there's several key differences in both context and in execution. For one thing, Belle is a relatively new character who's purpose in the story is still has yet to be revealed and that's something of an eventual payoff to her inclusion in the series. But much more important is the fact that said arc largely relies on Bell still needing to be realized and her having overt character focused is something that comes with different parameters that will lead to an eventual payoff: similar to Whisper and arguably Starline, she's not just a cool new action figure and has an air mystery that lends to gradually development in tandem with actual storytelling, with the difference being that the mystery itself is made fairly obvious and she doesn't have a grasp on who she is, what she wants, or even how much of the truth she doesn't have that they used to establish themselves with. Kinda like the Nobody who goes by Rey, she knew the truth all along, but desperately wanted it to be not the case and just needed something, anything to make her feel like there's meaning to her existence. But it doesn't help that as you said, Belle doesn't have an clear Finn, Kylo, or maybe Chewie(depending on how you view Orangutan Dude) to lean on in a meaningful way nor does she have much more characterization to drive her to leave an impact--in fact, she has less! She's just something everyone looks at and sorta recognizes for what she is, be it in seeing her as a threat, as someone who exists, or someone to dissect and she's sorta screwed up her chance to notice her clearest calling already.
She's Jewel except prominent, Starline sans meaningful influence on the status quo in both directions, Whisper without a more unique draw, and even Tangle without clear marketability.
That's basically it, yeah.