The individual chambers are called boxes - which can either be stitched through or, have BAFFLE BOX CONSTRUCTION, which means that there are walls in each chamber / box of about 1 1/4 inch. Or, just think of them as 3-dimensional, versus the stitched through comforters. More effort to manufacture? Absolutely. It requires quite a bit of extra work, hence down comforters with baffle box are considered higher quality.
It keeps the down from shifting and also allows the down clusters to fluff up when you shake out your duvet.
How many boxes are in each comforter depends on the quantity of filling. Summer comforters with lower quantities of down need more chambers so the down stays in place, whereas medium and warmer duvets use fewer cassettes. If you compare comforters form different sources, always ask if they provide baffle box construction. Although the construction might not be necessary or, even counterproductive in lightweight comforters, individual 3-D-chambers add to the quality of medium and, all year around comforters.