By stretching the film, the tensile strength of the film increases so that it becomes drastically stronger.
After the film is stretched it wants to return it to its original form. The more you use this pre-stretching process, the worst the film memory becomes.
When the stretch film is applied to a load with no force to load, quickly it will be able to detect that the film has tightened due to its memory as it tries to return to it original size.
Stretch film memory is what distinguishes stretch film from other means of packaging unitizing.
Settling that can happen during shipment can loosen other methods of unitizing products.
Stretch film memory takes up the slack and continues to secure the load during shipment.
Your cost of stretch film is reduced when it is pre-stretched compared to stretch wrapping a load with film that has not been.
Using just hand or machine to add tension to stretch film does not provide uniform stretch and is not able to have reliable performance. This type of stretching can lead to intermittent load failures.