You are correct that the cruise reps at the resorts are there to facilitate things for guests cruising on that trip. They are not booking agents. However, if you call "front desk" from any WDW resort, they can connect you by phone to cruise line reservations. There is no advantage to doing that from WDW over booking from home.
The cruise reps are stationed at the "cruise resorts" to be sure that the luggage, transfers, etc. go smoothly, to give directions to people driving to the port, and to generally answer questions about the cruise. They then be sure that all scheduled passengers are on the bus to the port, etc.