What if Cordelia's wish created an alternate universe where Buffy and Spike fell in love? 

In the episode, Cordelia tries to talk to Buffy, but a vampire attacks and only causes Cordy more humiliation. What if the vampire didn't attack? Cordelia and Buffy talk, during which Buffy states she knows how Cordelia feels; she knows what it's like to be betrayed by a loved one. 

This makes Cordelia realize 'Wait, what am I doing feeling sorry for myself when Buffy had it far worse?' Angel turned evil, which is far worse than what Xander did to her. While Cordelia is suffering, Buffy suffered a lot worse; for the first time in her life, Cordelia finds herself feeling sorry for someone else. 

This makes her think, and by the time Anya confronts her, Cordelia has figured out that everything went wrong because of Angel. Cordy chose to hang out with Buffy's group because they felt like real friends; they would never turn their backs on her, unlike Harmony and that lot. It was Cordelia's choice, and not a fault of Buffy's. Things went wrong when Angel turned evil, so Cordelia thinks that it's all Angel's fault. 

In short, she makes a wish that changes reality for the better - in that Angel is out of the picture and Buffy is in a relationship with Spike. How this happened is up to the author.

What else has changed is also up to the author.

If the necklace is broken, then Cordelia, Buffy and Spike somehow retain their memories from the other reality.