What is the main motive behind dealer incentives to

The motive is to entice (get, encourage) people to buy from the dealer.

The main motive behind dealer incentives is to give the dealers a low price for stocking the companies products. The company that gives the biggest dealer incentive will attract more dealers to actively sell the product of that company. It can be seen in case of cars, the incentives given by the manufacturers to their dealers for stocking the cars.