Masks are what make surgical edits possible: change the background and keep the product, change the jacket and keep the face, replace the sky and keep the roofline. Without one, every edit is a re-generation and every re-generation is a fresh chance to drift.
In a plate-locked pipeline masks do a second job. Masking the product out of the edit region guarantees the locked element cannot be touched by an inpainting pass, which turns a discipline into a mechanism.
Mask quality is the whole game at edges. Hair, glass, motion blur and semi-transparency are where an automatic mask fails and where a bad edit announces itself.
How does a mask help protect a locked product?
It excludes the product region from the edit, so an inpainting pass physically cannot alter it. That converts "do not touch the product" from an instruction into a constraint.
Where do automatic masks fail?
Hair, glass, motion blur and anything semi-transparent. Those edges usually need a manual pass, and they are exactly where a viewer looks.