The sales order origin represents the primary location for the sale.
When product for a sale is reserved (either by changing the sales order status to committed or by manually reserving items using the blue reserve links), the product is reserved at the sales order origin. The sales order origin may be set to the value "-- Unspecified --" in which case reservation for the sales order are not held at a specific location.
The sales order origin location name and postal address is printed on documents and reports based on the sales order (such as the PDF generated by the Print sales order menu item on the actions menu on the sale order detail screen).
The sales order origin is also the default shipment origin for any shipments created for the sales order (although this value may be changed for a specific shipment to handle the situation where a sale is fulfilled by shipments from multiple locations).