When you buy new tyres, you have depending on tyres, winter tyres will have 9-10 mm depth, and summer tyres will have 7-8 mm. When you reach 4 mm, you will start losing some of the key properties, and you should change the tyres to a new set. Safety when driving is correlated to the grip you will have so that you can have a short braking distance to avoid accidents, that you will have excellent grip on dry and wet tarmac so that you will be able to stay on the road while cornering. With a bad grip, you can easily lose control and start skidding. For more information regarding good wet grip, visit: https://lafromlasblog.com/why-tread-depth-matters-more-when-it-rains/