Outlining what is expected of a business helps to formulate a view of what you value and think about your business. Most successful businesses map out a plan or strategy on what they want to accomplish. Forecasting from a financial perspective demonstrates what you want to strive toward as a business and gives each department or group a target or benchmark to hit.
There are two main things you can achieve by forecasting, 1) the business grows financially and will attract investment and interest, 2) employees can set goals and gain a sense of pride and accomplishment when they achieve their target.
Forecasting can be a powerful tool on many levels, if used properly.