Ballet choreographers work habitually with other bodies - a huge shared vocabulary makes this easier. One is also often working with larger numbers in big spaces (and within the confines of a traditional hierarchical environment). When I began to choreograph I worked material out in advance of every rehearsal and would have found a computer useful in the way Joanne describes.
However, I increasingly leave the movement options open to emerge in the studio, even when working in a classical idiom - preparation is more a matter of defining intentions, compositional strategies and parameters. This way the choice of movement material is not bounded by the limitations of my own body and its habitual movement preferences(although they will inevitably colour the outcome), but benefits from the personal reaction and input of the dancers in the moment of creation. A movement phrase generated and structured through the computer might be wonderful on a formal level, but could it have the emotional charge and unpredictability of what is generated live in the studio?