Question

Why do car manufacturers offer dealer incentives?

Answer 1

When they have inventory they need to sell quickly, such as when the model year changes, and they need to sell the old model year's cars.

Answer 2

Using new car incentives to your advantage is a smart way to get a good deal. Most often offered through the manufacturer, an incentive can lower the overall cost of a car through a reduction in the terms of the lease or the finance interest rate. Although incentives are subject to eligibility requirements, if you have a decent credit score, it's possible to get a better deal.
The two most common types of manufacturer incentives are:
Customer incentives.
Dealership incentives.

source : http://www.dmv.org/buy-sell/new-cars/incentives.php