Typically, materials substitution is done for reasons of cost, performance and weight; companies are constantly looking for ways to make products lighter. Usually, a company would move from a metal to a plastic—the automotive industry has been doing this for years—but companies can also substitute materials that are produced using a different process.
For example, because the automotive industry has been moving from heavy steel to lighter plastic, the steel industry realized it needed to counteract this trend. So they developed a full range of new steels that, when produced in the right way, reduce the weight even further. Therefore, innovation can come from substituting material A for material B, or by taking material A and rethinking it and reprocessing it to achieve the specific improvements that are needed.