I don’t have a source to offer, but remember reading that the primary purpose for building, maintaining and expanding the canal network, all the way back to its origins in the 4th or 5th century BC was to deliver grain to the capital. So when the capital moved to Beijing, new sections were built for the same purpose.

So I would guess that these are general purpose cargo barges that could transport grain to the capital, and other commodities & goods (and political power) inland.

The canals go all the way down the inner coast. So could be any product of northeast to south east china