There is something special in the act of teaching somepony else how to fly. It is an integral part of an average pegasus's life, and how it is taught is just as important as the teaching itself.
Rainbow Dash doesn't just want Twilight to know how to fly. She wants to pass on something dear to her heart that was taught to her by her father. Rainbow wants it to be an unforgettable experience, and show why she loves the sky so much. There is so much more to it than mere wing flapping...