"Therefore, fulgurites may be ground up and added to spells in order to intensify effects and significantly boost their power." Buffy's past trust with Angel is added to her relationship with Spike to boost the significance and her ability to accept him and his intentions of redemption.
Created by Drizzlydaze, Foxstarreh, and Spuffy Luvr
One of the most commonly seen tropes in Spuffy fanfiction is the concept of how Buffy's previous relationship with Angel essentially ruined her ability to have a relationship with Spike.
Generally, the basic idea is that because Buffy's relationship with Angel ended in blood, death, creepy stalker drawing on her pillow, and her friends being hurt and murdered, she becomes closed off to the notion of a vampire being able to be redeemed without a soul.
And this idea isn't without base. The distinction between Angel and Angelus is enormous. They are wildly different people, and Buffy experiences just how much misplaced trust can backfire.
However, what ends up missing from the argument is the fact that Buffy started off not trusting Angel.
She makes Spike earn her trust just like she made Angel earn her trust.
In 1.07 "Angel," Buffy's response to discovering that Angel is a vampire is to go out, hunt him down, and try to kill him. There's not really any consideration that he could be good, or could become good. He just needs to die. Vampires are evil, full stop.
And it's not even after Angel explains that he has a soul that Buffy trusts him. It isn't until he saves her by killing Darla, the woman he was obsessed with for over a century, that Buffy gives him the benefit of the doubt. It is then that she decides to take his lack of attempts to kill her, along with his occasional help, to mean that Angel is 'good'.
Angel earns her trust much more quickly than Spike does. Not because Buffy hasn't yet learned to not trust people, but because Angel has, up to this point, been consistently on her side. Sure, maybe he's a little creepy and stalkerish about it, but Angel never, say, threatens to kill her on Saturday. Nor does he kidnap her friends and hold them hostage in a burned out warehouse.
If there hadn't already been a basis for that trust between Buffy and Angel, Angel very easily could have ended up dead that night at the Bronze.
Spike doesn't have that background when he decides he wants Buffy to trust him. You know what Spike did have, though?
Three years of trying to kill her and her friends.
Despite that, once he stops actively trying to kill her, Buffy doesn't actively try to kill him.
Her mindset has changed from 'vampires must die' to 'vampires who are killing people must die.'
Why? How?
Angel.
Yes, Spike has to work for her trust, just like he should have to, but Buffy is only willing to accept the idea of trusting him because of her past with Angel. Because it's already been proven to her that vampires aren't necessarily mindless monsters.
Because of Angel, Buffy has already made the paradigm shift that vampires can be allies. And so when Spike approaches her, wanting to save the world, she doesn't dismiss him out of hand. She listens. And she works with him. Sure, end of the world circumstances and all that, but a Slayer is trained to see vampires as the enemy. Always. She may not trust Spike, but the seeds are there.
When Spike becomes a more permanent part of her life, Buffy already has the idea that redemption is a possibility. Even if she actively denies it, she is aware of it. So each of Spike’s actions toward his redemption, toward earning her trust, actually becomes more significant because of Angel. Because his actions can be taken in the context of the possibility of redemption, Spike achieves Buffy’s trust much more quickly than he could have without Angel having paved the way.