The stress element represents a point on or in the structural component. If the overall structural component is in equilibrium, then any portion of that component must be in equilibrium as well. Consequently, the stress element must satisfy equilibrium.
Equilibrium involves forces, not stresses. Therefore, each stress must be converted to a force component from the product of the stress and the area that the stress acts upon. The forces are then shown on a free-body diagram of the stress element.
Since the stress element is infinitesimally small, we can assert that the normal stresses acting on opposite faces are equal in magnitude. Consequently, the forces arising from these normal stresses cancel out each other.