One advantage might be booking a guaranteed "category" versus a guaranteed room assignment.
DCL may have more "flexibility" to upgrade you to a better category.
I do not know if and independent TA can do this or not?
We booked through DCL, and recently got 2 of 4 rooms in our party upgraded from category 8 to category 6, and DCL reduced the price by $1700 so far (Florida residents discount). I'm waiting for the 60-day point to check on the other two rooms.