Simply knowing that demand for your goods or services is rising or falling is not itself. Knowing the rate of change is likely to be vital to business planning. A firm of lawyers may have to decide the point at which, in their growing business, they will have to take another partner. Hiring a new partner could take months so they need to be able to when they expect to reach that point and then when they need to start their recruitment drive. The same applies to a plant manager who will need to purchase new piece of machinery until equipment rising demand. She may not want to commit to buying an expensive it absolutely necessary but she will need to allow enough time to order the machine and have built, delivered, installed and tested.
The same is so for governments planning new airports or runway capacity or deciding where and how many primary schools to build.