The slick has no grooves which means more rubber, the bigger contact patch with the road. More rubber translates into more traction and this means the vehicle has more grip and a greater percentage of the available output power is transferred by the wheels. In other words the slicks stick to the road better and gives better acceleration. The lack of treads means that the slicks would deform less under load, so slicks can be made of a softer compound than a treaded tyre for the same vehicle.
The main disadvantage is the susceptablity to aquaplaning, no running on wet or water clogged tracks. Since slicks offer greater resistance, it will tend to be slower in a straight line than a treaded tyre of the same dimensions.