Private, off-airport lots sometimes offer extras which can make them more convenient and a better deal than airport parking.
They compete on price, to be sure, but they also compete on service (such as luggage assistance and direct car-to-terminal service) and extras.
Some lots offer oil change, carwashes, detailing, newspapers, bottled water, and frequent flyer miles. One lot in Atlanta even offers pet boarding.
Private lots also tend to adjust their prices to fill up spaces during less busy periods, offering coupons and online discounts.
Traveling during peak holidays? Consider making a reservation — something unknown at public airport lots.