Interfacing being sewn into the left side. It covers the Pleats in the back at top of the outer apron. Humans exert approximately 44 pounds of pressure per square inch.
Reference: Forensic Biomechanics
Why this is important to you? If a "Average" Male has a waist of 36" inches and the Kilt has a 4" inch Band. 4 * 36 * 44 = 6,336 Pounds per square inch! (It goes up for bigger guys...) SO, if you are wearing a kilt that does not have interfacing AND you are blowing out the seams. This is why.
Because of this Interfacing is brought all the way around the kilt. In the photo below you can see the belt hole being sewn completely around with the interfacing
Lining pinned and ready to be sewn.
Right side apron with interfacing sewn in place.
Lining pinned in place ready to be sewn.