A down payment is used to pay off part of the car, and then you finance (get a loan) for the remaining amount.

If you paid the dealership $5k for a car worth $15k, but then get a loan for $15k (which they wouldn’t allow anyway since you have a down payment), you would end up paying $20k (not including interest) for a car only worth $15k.

So it’s better to give them a high down payment so you don’t owe as much on the loan.

The loan has interest too, which means even if you get a loan for $10,000, you will end up paying more than that depending on the interest rate and the length of the loan.