I think that Hannah's (not Hannasciel, but Hannah Ascher herself) main purposes covered a fairly wide range.
1. In-universe: Harry's preferential treatment toward women is documented and referred to by villains and allies alike, at least three hundred and fifty-eight thousand, two hundred and twenty-seven times per book. Nicodemus sought her out for a couple of reasons, including her natural talent with fire. The other is to provide a hole card for the inevitable confrontation with Harry, and picking a pretty woman is a good way to catch Harry off-guard.
2. Narratively: Hannah's kind of a dark reflection of what Harry might have become. Warlock who killed people in self-defense, but with no Ebenezer to take responsibility. Ran from the White Council and lived as a fugitive. Sought protection and an alliance (and, from her words, friendship) with the Brotherhood of St. Giles. Still hunted and without an organization to protect her, she joined up with Nicodemus.
3. She also represents one facet of the direct human consequences of Harry's decision to kill the Red Court. She put a face to all the faceless people who have suffered from the fallout of Chichen Itza—not just the defenseless Paranetters or plain ol' mortals who've been attacked or abducted by the Fomor, I mean. She can defend herself (quite well, I'd add), but she's one of the people who lost "everything" because of Harry's choice (not quite everything, obviously, since she's still living and breathing.)
4. Her righteous indignation was the result of circumstances that were similar to Harry's; if he'd either A) not had Ebenezer to take responsibility for him and guide him, or B) simply been more bitter over his situation, he very well might've joined Nicodemus way back in Death Masks. The difference is that Harry had good people in his life; Hannah didn't. Or she did, but they were all killed.
What would've happened to Harry if everyone had died in Changes? He might not have picked up a Coin, but I'm willing to be he'd be a lot more like Mirrorverse Harry come Skin Game.