A new tyre consists of the casing or the body and the tread. The casing is an incredibly complicated flexible pressure vessel. It is designed to withstand high temperatures, severe impacts from the road, and heavy pressure. 70% of the cost of the new tyre goes into making the casing. Only 30% of the cost goes into the tread. If you throw out a tyre when the tread is worn, you are wasting 70% of your investment. By retreading, you save this money.