It helps stiffen that part of the garment. It is a usually white material sold in different weights or thicknesses depending on how much body you want to add to the garment. Usually used in collars, around a neckline, or in cuffs, maybe a waistband. You do not have to use it but it can make the collar or whatever look more polished. If your fabric is not very limp, you probably do not need it.
You also use interfacing if you are going to have a button up closing. You put a layer between the two fabrics. It helps the garment to look more professional.
I have looked on youtube but it does not tell me what it is used for in sewing. what is it for and do you have to use it?
