Sofas are usually built to one back height, which means they suit one way of sitting. That is fine for a two-seater, where everyone is doing much the same thing, and it is a real limitation on a long sofa where four or five people are using it simultaneously for different reasons - one reading upright, one watching a screen, one half lying down.
Stepping the back solves it without any mechanism. Different heights along the same piece give different support, and people find the section that suits them without being told.
It is more work to build. Each panel is framed, padded and covered separately, and the transitions between them have to be resolved cleanly or the back looks like a mistake rather than a design. Our master Indian artisans overlap the panels slightly at each step, so the taller one always sits in front of the shorter - which is what makes the change read as deliberate from across the room.