As a SAS consultant I've worked with external forecasts in SAS Forecast Server on a number of projects. The main reason has been the company's request to include forecasts generated the way they used to be generated into the forecast flow provided by SAS Forecast Server. For instance, if the company historically has calculated forecasts as simple averages of indexed time periods (the forecast for January is the average of actual values from previous years January) including these as external forecasts in SAS allows you to compare them to forecasts generated by exponential smoothing models, ARIMA models and UCM models.
Please let me know if you have further comments or questions.
Cheers,
Snurre
