The question would be more specific I guess. Since forecasting can be for sales planning, production planning etc
In simple terms forecasting creates a realistic goal for your organisation through historical data and your planning is then streamlined to meet that goal.
So if a hotel in mahabaleshwar is empty every year in the month of august, it will remain empty even this year in the month of august and the hotel owner can now do 2 things,
1.) Forecast low revenues for that particular year, keep his variable costs at low, probably cut down on lavish meals in the hotel for that particular season etc etc.
2.) He can create a promotion of 50 % discounts etc which will help him increase his hotel occupancy rate and eventually revenue.
These were simple examples, of forecasting. You can read more technical theories on