20 years ago when I lived in Champaign-Urbana (2 trains each way in those days) the station was staffed from about 5 am to midnight. I bought most of my tickets from the agent there, or reserved them over the phone and picked them up there, and I often saw someone else buying a ticket at the same time. I collected express packages there once, picked up delayed luggage once, and checked bags early once (when that was still allowed). I think the agent also printed out last-minute phone-order tickets and handed them to the conductor to have them for passengers with reserved tickets boarding at unstaffed stations further down the line. Just before I moved away, I think, Amtrak cut one of the agent shifts (the evening one), so there is now only checked baggage heading north, not coming south.
Handling baggage and express is still important. Marshalling 80 or 100 people onto the Illini, Saluki, or City of New Orleans, and getting two or three carts of checked bags out to the platform and onto the train, is not trivial work. Even at a one-each-way station, getting a bunch of people and their bags off and on might still be worth it. Some people probably still buy tix in person. But I assume that pdf printable tickets and smartphones have made agents a lot less necessary.