It provides a surface for catching hand stitches (like hems or facings) so they don't show on the outside of the garment.
Popular choices for underlining include silk organza for crispness, cotton batiste for soft structure, or muslin for general stability. 2. Underling in Pop Culture How to Sew an Underlining - Seamwork underling
It can hide inner construction details or prevent semi-sheer fabrics from being see-through. It provides a surface for catching hand stitches
Instead of sewing a separate "inner garment" (lining), you cut your pattern pieces out of both the fashion fabric and a backing fabric (the underlining). The two layers are basted together and treated as a single piece of fabric during assembly. Purpose: cotton batiste for soft structure