Linear TV is a futures market: deals are agreed ahead of campaigns being aired, usually with an agreement that a particular audience level will be delivered. For this to work as efficiently as possible, good audience forecasts are essential. Where forecasts are wrong, the media owner either over delivers, effectively giving away audiences for free, or under-delivers, requiring additional inventory to be added to the campaign for free.
Forecasting is a lesson in humility because no one gets it right all the time (Exhibit A: the media’s current favorite geeky guru, Nate Silver, who failed to predict that Donald Trump would be the Republican presidential nominee). No forecast is perfect and no forecasting model is good for all time, because the landscape is always changing.