Back in Equestria, Princess Celestia had the job of raising the sun. And as such, it was believed that without her power, the sun would never come up, and there would never be day, just like how without Luna, the moon would never come up, and there would never be night.
Sunset Shimmer thought this was the rule in all worlds... Until she was told the Equestria Girls world was different.
Still, she needed proof, so she took it upon herself to verify if the sun really rose in this world without Celestia's assistance.