Question

why do you need to change the tyres when it's threading is finished​

Answer 1

Answer:

When tires become worn there are very little tread depth left for water to be pushed out and allow the rubber to contact the asphalt surface you lose friction which you depend on for traction to enable you to accelerates turn and slow down.

Answer 2

Explanation:

Once grip is lost, the amount of friction between the tires and road drop by about half (which causes skidding and hydroplaning). Lastly, once the tread is gone, all that is left is the rubber over the belts. Once that wears through, the tires will literally come apart while driving, throwing pieces all over the road.