As Jujutsu Kaisen fans must remember, after Mahito transfigured Kugisaki Nobara's face, they spent a long time waiting for confirmation of her status. While Arata Nitta stated that he managed to make her chances of survival "non-zero," it took another 140 chapters before the manga showed the series' tritagonist making her return.
While many fans believed that the character should have returned much earlier, others felt differently. Nevertheless, the truth is that manga creator Gege Akutami did not delay Nobara's return for a specific plot development, but because that was their only choice.
Disclaimer: This article contains spoilers from the Jujutsu Kaisen manga.
Gege Akutami delayed Nobara's return to save his plot in Jujutsu Kaisen

As seen in the manga, while Nobara Kugisaki was absent from the series for a long time, she was never dead. This meant that the manga creator never intended to kill her, but rather to make her unavailable for the plot to progress as they envisioned.
As fans must remember, the creator had faced criticism for sidelining Satoru Gojo for plot reasons. Given his status as the strongest sorcerer currently, if he had been operating as usual, much of the plot would not have unfolded as it did. Therefore, Gege Akutami needed to introduce a plot device to keep Satoru Gojo out of the story.

Evidently, the same was true for Kugisaki Nobara. As seen in the manga, on December 24, 2018, Nobara recovered from her injuries, and in less than an hour, Utahime and Gakuganji brought her up to speed about the battle against Ryomen Sukuna. Soon after, Nobara was taken to the headquarters where Sukuna's last finger was bound.
This development changed the tides of battle in favor of the Jujutsu sorcerers. Just as Kugisaki Nobara activated her Resonance cursed technique and struck Sukuna's last finger, the damage bound the King of Curses in place, allowing Yuji Itadori to finish him off with some help from Megumi Fushiguro.

This development only worked because, like the audience, Ryomen Sukuna was unaware of Kugisaki Nobara's status. Had he known that the Resonance cursed technique user was alive, he would have been more careful with his cursed objects. Unfortunately, much like the audience, he believed she had passed away and let his guard down.
Therefore, while it may have seemed to the audience that Kugisaki Nobara was excluded from the plot because her Resonance could have easily turned the tide of battle in favor of the Jujutsu Sorcerers, the truth is that the plot point only worked because Sukuna wasn't expecting it. Had Nobara's return happened any sooner than the start of the Shinjuku Showdown Arc, Ryomen Sukuna might have devised a counter to the Resonance cursed technique.
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