Paying cash may not get you as good deal, as some offers are based on getting finance from the dealer. Also HP also has the added benefit of extra security. The loan is secured against the car, not you. Which also means that half way through the term you can give the car back, Scott free and no mark on your credit record. So if life takes an unexpected turn, you can get out quicker to reduce personal expediture. This happened a lot in the 2008 crash when people were paying more on a car than what it was actually worth.
On my wife's fiat 500 we got £3k off the price if we took the dealers 0% offer but they would only do a £500 discount for cash. Stupid to me but they have their quotas to meet I guess. We were going to just pay it off after the first month but as it's 0% we may as well get the interest on the money in the bank.