When ArcGIS Server executes a task to a Geoprocessing Service that outputs a layer, it reads the selected features from the layer and transports the selected features back to the client. Outputs are drawn by the client. When you publish with a result map service a Map Service is published in tandem with your Geoprocessing service these two services execute independently of one another. When the task executes, ArcGIS Server executes the geoprocessing task first, then executes the result map service, which draws the output of the geoprocessing service, sending the map image of the output data back to the client.
You would choose to publish with a result map service if:
The selected set of features can be large and you do not want to transport a large number of features back to the client.You want to protect your data and only let the client see a map image of the data or You have advanced cartography that can only be drawn by ArcMap, not other client applications.
Where does the execution of your GP service come into play with this workflow? Are you executing the GP service through a Geoprocessing Widget and then hoping that the data in the result map service can be read by the chart widget? Can you confirm in itself that when you run the GP tool that it updates the result map service? Or is it always displaying the output from when it was published?
