In order to have traction in the rain, tires need to be able to channel the water away; that’s why there are grooves in the tread. Rain tires, such as the Michelin HydroEdge, have one or two large grooves running around the tire for that purpose. By channeling the water away, the tread can make contact with the road (this is good, as it allows steering and braking). In contrast, a well-worn tire doesn’t have deep enough grooves to do this, and so can ride up on the water. This is called aquaplaning or hydroplaning (not good as you won’t be able to steer or brake).

In short, it’s not worth the risk by driving around on worn-out tires. Sure, you may not notice it if you mostly drive on dry roads, but when the rain comes and you’re sliding all over the place, you’re gonna wish you replaced those hoops.