Is there really a difference? dealerships buy the vehicles at a small discount, pay interest to ford/gm/etc. the dealer also foots the bill for rent, employees, keeping the lights on, etc etc
Independent dealers are a separate entity buying the car on time from the maker. If sales go soft for a short period, the dealers take the hit and the manufacturer is shielded from the downturn by the dealers. Some dealers may go bankrupt, but the manufacturer just takes back the cars they already made some money on and sells them on to another dealer. The manufacturer will be in trouble if the downturn lasts longer as their buffer will dry up as dealers fold in ever larger numbers. At the end the failure can become catastrophic as it did for GM and Chrysler and almost did for Ford in 2008-2009.
Without separate dealerships, the manufacturer feels any downturn in the market much faster.