You want a good tire contact patch with the road. And, that depends on how much air is in the tire. If too much air pressure, the center of the tire is carrying the brunt of the load, and the grip diminishes.
With too little pressure, the center depresses upwards and the contact patch becomes the two edges of the tire.
With the right amount of air, for the weight on the tire, the contact patch will be the maximum. More rubber on the road means better handling, better stopping (emergency) and better tire life.
2008 Monaco Camelot 40' PDQ