Practicing spi balls is a method of energy manipulation, as others have said. It's a means of sending your own energy in a specified direction, or back, pooling it in a place, controlling it, programming it, changing the "type" of energy, and so forth.
As far as actually seeing a psi ball, you will likely never see one. However, this can also be an exercise in visualization. You can visualize the energy between your hands. Some say it helps them direct their energy and intent. For example, if you are pooling a healing energy between your hands, and to you the color sky blue is associated with healing, then visualizing a sky blue energy ball, as solid, misty, swirling, or whatever, as you want it to be, making it as real in your mind's eye as if it were really there, will make the exercise more real to you.