Generally, when your car needs new tires you look for something with good tread so you have good traction with the road (unless you're broke and your only requirement for tires is the cheapest thing they sell at Walmart). But drag slicks are just totally smooth, so why do they hold traction so well? Well the answer is really simple actually, it mostly comes down to surface area. Since slicks are perfectly flat, the contact patch the tire has with the road is much larger, providing better traction. Also, since there is no tread on the tire, there can be no uneven tread wear, so they can be made of a softer tire compound that has a larger coefficient of friction with the road, providing much better traction than standard tire material. So, if these tires provide such great traction, why don't all cars use them? Well, the answer, again, comes down to surface area. Since essentially the entire tire contacts the road, it is very impractical in snowy and wet conditions, as more snow or rain gets under the tires, it has less and less contact with the road, and will begin to either slide or just spin. Meanwhile, conventional treaded tires can push the snow or rain into the grooves, and maintain contact with the road, keeping traction with the road.