Lily Beat

My theory is this: In The Cutie Mark Chronicles, after Twilight's "magic episode", Celestia saw how much power Twilight really had, thus trained her to be her student. Something similar happened with Cadance. (From Wikipedia: "When she grew up she reversed a love-stealing spell cast by a pony called Prismia. Prismia wore a magic-enhancing necklace that amplified her evil and jealousy, but when Cadance confronted her, the necklace amplified Cadance's love. Once Prismia changed her horrible ways, Cadance was surrounded by magical energy and transported to a strange place where she met Celestia, who adopted her as her royal niece.") Therefore, Celestia saw something special in both of them, so they both have some sort of special power that Celestia and Luna may or may not have. And of course, Equestria needs them because of that.