Well one reason is because that means that the tires are thin and can pop easy and cause a flat tire or even an accident if you are driving fast. Another reason is that there is no traction and if it is raining or snowing out and you are driving this could cause your car to lose control and spin out and even crash.
the tyres get worn down and it becomes dangerous because the tyres have no gripBadly worn tires can blowout and you can easily loose control of your vehicle.
A bald tyre is a tyre whose tread has become worn down to a smooth or nearly smooth surface. A bald tyre is extremely dangerous as grip on the road surface is reduced and skidding is a probability. In the UK, it is illegal to have bald or nearly bald tyre(s) on your vehicle.
Tires are changed during an auto race because the tires get worn down by the track. It is safer to have good tires than to have tires that are worn down that could blow apart and cause accidents.
Conglomarate
Winter tires should be changed out when the tread has been worn down to around 1.5mm to 1.6mm in thickness, after they have been worn down more they are considered worn out. To measure the tread of ones tires one should use a tire gauge which can be bought at websites such as amazon.
Tires with worn tread will hydroplane easily, very dangerous.
If you're having to put air in your tires frequently then your tires probably need replacing. Also if the tread on the tires is worn down they should be replaced (a symptom of worn tread is the tires squealing on the road for no apparent reason).
All tires have a wear indicator. Look on the tire for that indicator between the tread. Once the tire reaches that point they are worn out and need replacing. Another test is to insert a penny with Lincoln's head down. If you can see the top of Lincoln's head they are worn out.
Tires have molded indicators called "wear bars." If these are visible across the tread pattern, your tires need to be replaced.
Smooth tyres have less frictional grip on the road surface, and will slide further that a deep tread tyre.
Worn tires are worn down to 1.6 mm of their remaining tread depth. They could have bulges, bubbles, slits or holes. For safety reasons they should be changed.
If you drive with low tire you can have a blowout, which can lead to an accident. This is dangerous to both you and other people on the road.