It adds to Arcee's history. We already knew she'd been experimented on by Jhiaxus, so we already knew she was a contemporary of Galvatron. Being a sibling of his helped explain her immense combat prowess. That Arcee stood aside and let her brother die, evil as he may have been, helped her appreciate those she had around her more, leading to her relationship with Aileron. And finally, partaking in the early crimes of Cybertron and trying to move past them tied into the themes of the whole run.
It added to Arcee's character. If you didn't like her, that's fine, and if you didn't like the idea of her and Galvatron being siblings, that's fine too, but it did impact her story.