Why do tires have tread?
Short answer
Grooves in a tire give water a path out from under the part touching the road. This helps the tire hold the road on wet surfaces and makes slipping less likely.
How the grooves work
As a tire rolls over a wet road, water can collect under the part touching the ground. The grooves guide some of it out, much like channels that carry rainwater away. They cannot remove all the water. Tread patterns vary because they also affect braking, traction, and stability.
What happens when tread wears down?
Shallow grooves move less water. If a car moves fast over standing water, the tire may ride on top of the water instead of gripping the road. This is called hydroplaning. The driver may find it harder to steer or stop. The risk depends on speed, water depth, road surface, and tire condition.
What to remember
Tire grooves help on wet roads, but they cannot guarantee a safe drive. Slow down in rain. If the grooves look worn, ask a tire shop to check them.