Oddly enough I've used that site a lot and thought of asking here before looking on there. thanks Tors!
The answer appears to be as follows: -
(1) Local & regional trains in any country = turn-up-and-go ticketing: Tickets for local, regional & suburban trains can easily be bought at the station. The price is fixed, so there's no price advantage in pre-buying. Reservation isn't compulsory, indeed it's usually not even possible, so these trains can never sell out, so again there's no point in pre-buying. You simply buy a ticket at the station and hop on the next train, sitting anywhere you like. Think New York Subway! Buying online simply saves a few minutes at the ticket office, that's all.
Domestic trains wholly within Switzerland, the Netherlands, Belgium & Luxembourg work on this principle, with reservations neither necessary nor possible, even for longer-distance InterCity trains. You turn up, buy a fixed-price ticket at the station and hop on the next train.