Because this vision of herself in Celestia's dream showed (and I mean FINALLY showed) a darker side of herself. Not only was it a forcibly humbling dream but it was a pivotal moment in the episode where Celestia had to be reminded that deep inside, despite the temporary grudge she was having against her sister, that she loves Luna very much and this Daybreaker vision needed to happen in order to restore that relationship. Sure, so far it's a one time thing but that one time said a lot (IMO). And if you think deeper, this may have made Celestia think about what Luna went through as she became that entity named Nightmare Moon: being alone, nobody to talk to most of the time, and letting something angry and evil manifest within. It kinda put Celestia into Nightmare Moon's shoes for a short time in that dream. And of course, all this was part of that high-level friendship problem Starlight had to deal with. That's what's special about Daybreaker to me: it finally showed a side of Celestia that I think needed to be addressed. She ain't perfect, but she's still a great princess.



Hope this answers your question. Mind you most of this just my interpretation, not necessarily "fact".