When the tread wears down on a vehicles tires it means that the tires won't be able to keep traction on the road and especially in snow, ice and rain so it makes it harder for the vehicle to keep moving down the road safely.
The loss of tread and traction also means it makes it harder for the tires to grip the road to stop.
The tires tread is not only for giving traction to go down the road safely but also helps and allows the car to stop and not just slide when you press the brakes.
When the tread gets wore down and there's not much tread left then it's time to replace the tires regardless of tire age.
If you continue to drive on bald tires then the steel wire that holds the tires together can start showing through and then eventually the tire could blow out on you at highway speeds which can then lead to a deadly or serious injury roll over accident.
So replace those tires when needed.