The reason NASCAR tires don’t have tread is simple: grip. The tires you find on street cars have tread to provide grip in wet conditions. The tread channels water away from the tire’s contact patch. In fact, NASCAR indeed has a rain tire that is used for the Nationwide Series. If our dry-weather tires (also known as “slicks”) were to have tread it would remove rubber that could otherwise be making contact with the asphalt and provide more grip. Some racing series, such as short-track divisions and Formula 1 purposefully add tread or grooves to their tires to slow cars down in the turns.
If it didn’t rain, your tires wouldn’t have grip either. That’s why it’s very important to keep an eye on the tread-depths and tire wear on your street-car. If you were to hit a wet patch with no tread it could spell disaster!