Ok so treads are for dirt, snow, water, pebbles, etc. they give those particles a place to go when the tire rolls over them, So the tire itself doesn’t lose grip. Race cars during history tried to minimize the tread as much as possible to provide more surface area and less grip. Someone had the idea to just have one tread: the tire itself. It provides the most surface area and is perfect on the relatively clean race tracks. On the road though, these slicks would just roll onto water and debris, and would never make contact with the road, no grip! This is why when it rains, most Motorsport series switch over to different tires with plenty of tread to move the water out of the way. Otherwise it would just roll onto the water, not the road.