I am no expert on her playing habits, but it seems she uses it for encores or other situations where she picks a piece to play on a whim. I understand she has performed entire (solo) concerts where she left the music selection up in the air until the last moment, and that page of the program would literally be blank. I have also seen this comment elsewhere:
Throughout the 20th century and right up to the present, as contemporary music has become less conventionally melodic or harmonic, more complex and harmonically dense, and frequently atonal, it has become more common for musicians, including soloists, however photographic their memories, to play with the printed music.
I would also love to know how she plays in 6-inch spiked heels, but I understand any questions regarding her attire, including shoes, pisses her off...