Between compartments without baffles, where the top and bottom fabrics are sewn together, there is not a lot of room for down. There is therefore a region without insulation, creating a "cold spot." With a baffle construction, down is able to fill the compartments on both sides of each wall, leaving no area for heat to escape.
Whether duvets have boxes or channels, they are either sewn directly through from the top fabric to the bottom, or sewn with Baffles. A Baffle is a piece of fabric sewn in a perpendicular manner between the top and bottom fabric of the shell, similar to a wall, between compartments, creating a three-dimensional space.
Baffles and the resulting three-dimensional space allow down the room to loft up to its maximum potential. This in turn traps more air, providing better insulation and more warmth.