Not always, you only do backcrosses when you are trying to replicate one of the parent plants in seed form.
In a backcross, you combine two strains, one strain(Friesland in this case) whose traits you want to replicate in the offspring, and another strain(Pluton, from what I've read) that is usually chosen because it produces traits that are easily identified and thus easy to breed away from. From the new cross(Freezland) you then chose offspring that resemble the original plant with the desirable traits and back cross those to that desirable parent plant(Friesland) until you get seeds that produce plants that mostly resemble the desirable parent plant, creating an IBL . The point of this form of stabilization is to create a seed line that produces plants that resemble one of the parent plants as closely as possible, thus creating an IBL.
Or, you can cross two parent plants that both have traits that are desirable, with the intent to create a plant that carries the desirable traits from both parents, ideally resulting in something better than either of the parent plants. You then cross the desirable offspring from that cross that represents the new better hybrid, over and again(F1, F2, F3, etc.), until you stabilize those traits. In this scenario, there is no backcrossing to either of the parents because you're not trying to recreate either parent, you are stabilizing the traits of the new hybrid.
