The Siddhis give us the power of clairvoyance, levitation, shape shifting, invisibility and more. Imagine! Invisibility, how many ways could that prove to be useful? The Siddhis come to us as we practice Samyama; deep contemplation resulting from moving beyond Samadhi. This practice moves us inward and as we delve deeper into ourselves, we get closer to Yoga. As we move closer, our connection within connects us with those around us, too. Thus, we develop abilities that show our linkage.
Patanjali warns us, though, that the power of the Siddhis can be corrupted by the ego. That by using these powers we could end up living with more suffering rather than freedom. Just as with every aspect of our practice, the goal is not the power, not the beauty, not the impressive stuff that we can show off. No, the goal of our practice is to move beyond those aspects. Even as we acquire the Siddhis, we are not to use them but to renounce them. The goal of our practice, after all, is Yoga.