When tire tread gets low you lose the traction from your tires so it can be hard to stop faster and you might slide on wet pavement.
The tire tread on the tires is what helps the tires grip the road and also helps you stop.
Without proper tire tread your car or other vehicle would be harder to stop so if your tire tread is wearing down you should replace the tires.
If you stick a penny in the tires tread and see the top of Lincolns head on the penny then the tire tread is worn down enough to need replacing.
Also if the tires are 10 years old or older you should replace the tires with new ones regardless of tread depth because the rubber cracks and the steel belts separate in the tire which can lead to a blowout.
Most tires last around 50,000 miles before needing replaced due to tread depth however if the tire is 10 years old or older it should be replaced regardless of tread depth.