naomi misora's function as a plot device is very important to death note for many reasons:
closest call light has had up to this point. extremely suspenseful episode/chapters
it was hinted at with light staring raye down in the subway but naomi's death drives in how much light loves telling people he's kira right before they die which is a critical character trait right up to the end
furthers the themes of suicide and suicidal ideation that death note so loves
first (only?) person to call light and L similar out loud to light's face
first person to have a direct connection to and grief for one of kira's victims. light and L's points of view are extremely narrowed in on the game and naomi forces the world to widen and show that the mass murder has actual impact on real people caught up in their wake
first person light has to use significant social manipulation on - i'm not counting yuri because she wasn't the component on which his spaceland plan rested. demonstrates for the first time the extent of light's ability to mask as boy-next-door
most sympathetic victim yet. was even less of a direct threat to light's life than raye was, plus she's acting out of grief for her fiance. demonstrates for the first time the extent of light's cruelty
i also think naomi is a fantastic character in and of herself but she is a critical part of death note's arc as well imo. her existence might not contribute much to the logical deduction side of the plot progression (though her disappearance is the final straw that gets the task force + L to focus on the people raye penber was stalking) but is so so important for the development of our protagonist and the establishment of the world around him that taking her out would not only remove one of the most iconic scenes in manga/anime but also destroy a HUGE amount of the subtler thematic and character work.
