Because she's a dancer, her feet need to be defined.
She cant just have stumps or ovals for feet, she needs her heels to define her movements and better express her gracefulness. With heels and defined feet she can point her toes and stand on tip toe and better show momentum in her animations.
Hard to point your toes if youve just got stumps and heels would be better than flats because they give her better posture.
Plus her animations arent too complicated, having heels gives her feet a definitive shape. Were she wearing flats she'd need extra animated joints in her feet to compliment her dance moves. Heels negate the need for the extra work.