I can't speak to all products as they will vary, but with our Detail Spray theres a good amount of benefit to be had by drying with it. Most sprays should offer you these benefits, but there are some truly horrible products out there on the market so not all of them.
- Its hydrophobic - the chemicals in our Detail Spray don't mix easily with water... spraying it as you dry will cause the water to roll away as you spray, making drying a little easier.
- Surface lubrication - anytime you touch your finish, even with a high quality drying towel, you want to add some lubrication to the surface. It'll help minimize the chances of swirls being introduced while you dry. Also should a rogue bit of dust/grit get onto the finish either due to a breeze or some suspended in a drip of water coming out of a door handle the detailer will help lubricate that so its safe for removal.
- Shine - it doesn't leave as much gloss behind as when its used straight, but our product has a gloss enhancer in it. The portion that is deposited on the finish will give you a small amount more gloss.