Imagination is in the mind of the beholder
In the yoga sutras, Patanjali identifies 5 types of thoughts in an effort to zero in on the ways in which our minds can be distracted. In addition to naming them, he also says that these 5 kinds of thoughts can be harmful or non-harmful. In this way, practitioners can begin to curate those thoughts for the purpose of reducing those that cause harm. One of those types of thoughts is imagination.
Imagination is a type of thought that convinces us what we create in our minds is real. We only have to close our eyes and imagine doing something that terrifies us to know that our mind/bodies don't differentiate between real and imagined thought. However the thing that Patanjali doesn't address is that imagination can be helpful as well. The power we observe in our minds when imagining terrible things is the same power we can harness to imagine wonderful things. In fact, I would say that it's impossible to accomplish something unless one can imagine accomplishing it first. Otherwise how would anyone begin the process of achieving any goal? It is essential that we allow ourselves the ability to envision ourselves in a certain light if we hope to make that vision a reality.
On our yoga mats we use our imagination all the time. We "imagine" breathing into our hips and we can feel that image become a sensation. We "imagine" growing taller in a posture and we stand up straight. We "imagine" our foot is anchored into the floor and we create stability. Most of what we do on our mats is first created in the mind since we rarely have the chance to see ourselves visually. Instead we "see" ourselves mentally. So imagination is a vital element in the successful practice of asana.
In life, we will never grow out of our comfort zone without first having the vision to do so. If we can imagine it, we have a chance to execute it. Without the gift of our imaginations, we cannot see past what we already live and who we already are. Patanjali is correct- thoughts can be harmful or non-harmful. But they can also be useful and even essential to our well-being and evolution. The mind is a powerful thing that can be used for ill or good. How we use that power makes all the difference in what we are able to achieve.