I've been buying a bunch of my TGV tickets online in advance...but when I'm going from, say, Annecy to Beune, or Chamonix to Annecy, is there any reason/recommendation to buy in advance?
For regional travel in France I don't believe there is any advantage to buying in advance, unlike the TGV.
Robert is correct. There is open seating on regional trains and no reservations are possible. Your ticket is good on any train. There are no discount fares.
If it indicates TER on the scheduloe, it is a regional train. Non reserved, but you must validate (compostier) the ticket in the machine in the station before boarding first train.