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.
Baffles
Whether duvets / comforters 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.