Assistance with understanding the forecast function

Please could someone tell me advantages of using Forecast function, and also what you would recommend as the best function in excel for forecasting sales affected by seasonal factors. For example I had 3 years sales where year 2 grew by 30 percent, year 3 grew by 32 percent but the forecast function only grew by 23percent in year 4 (x) why would that be? I am using about 5 products and I smoothed out seasonality by predicting year 4 in Jan, Feb etc

Looking for the best way of predicting but appreciate not an exact science.

Thanks

There's a workbook at https://app.box.com/shared/5gi9g8x23p that shows how to calculate seasonality.

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