Why is supply chain planning such an important activity? because it managers improve flows and control cost because it makes the supply chain reactive and unstable because it guarantees forecasting and inventory accuracy because it provides "just-in-case" inventory and excess capacity

Because it helps managers improve flows and control cost.

Explanation:

A supply chain is almost never perfect. Actually, it is more likely to be unstable and unorganized making the whole process inefficient. In order to prevent this, there are supply chain planners that are responsible for controlling inventory levels, forecasting sales and demand and, in general, optimizing the process of taking the goods from production all  the way to the final customer.

It is unlikely that a company can eliminate all uncertainty and guarantee accuracy in the forecasts and inventory. But it is an essential tool in order to have a better control of costs and flow of the information, financial resources and of course, the products.