The addition of the barbell in a front-racked position will call on your spinal stabilizers and erectors, the erector spinae group, to keep you from bending over forward. As you start to step up onto the box, your torso will naturally lean forward, but your erectors will have to contract isometrically (without movement) to keep you upright.
This "anti-flexion" movement is argued as the better, or at least the preferred way to strengthen the erectors, specifically in the lumbar region. We used isometric holds regularly in physical therapy to help strengthen any muscle that needed it- it was an important component to the strengthening process. Give it a try when it fits into your workout and let us know what you think!