> It also feels different from say the optional (and way lower priced seat reservations in German ICE's)

One has to add that most of the time when you buy a single ticket in Germany you have to (pay through the nose and) buy a ticket that is good for one specific train. Miss that train and your ticket is gone. This used to be different before the reform of the Deutsche Bahn after 1989; you used to buy a ticket for a 'communication' (connection between endpoints), not a specific train. TBF in like up to 200% of all cases Deutsche Bahn will route the wrong train in the wrong direction to a wrong station hours after the schedule in which case your Zugbindungsfahrausweis will fall back to work as a pre-reform ticket valid on all trains in that direction (sometimes even including an upgrade if an ICE is all there is, I think).