In sample and Out Sample Forecasting

[brianaus86]

Hi there,

I am confusing about when to use in sample and out of sample forecasting in Eviews.
Assuming that, I already have a fitted line with my data (Real GDP, 1950 - 2000, annually), as the equation is: GDP = c + b (Time).
The tutor told me that we need to use in-sample forecasting from 1991 to 2000.
I am confuse WHY we need to use in sample forecasting, when we already have the fitted line?
Thanks,