For NSB lokaltog such as Trondheim Lufthavn–Lademoen, is there any benefit to buy tickets in advance? At nsb.no there is only Ordinær togbillett (no Minipris), and there don't seem to be seat reservations either. Can I buy exactly the same ticket for the same price from a ticket machine at the train station?
If you are sure you will travel that day, you can buy train tickets online. That will help you if your flight is late and/or there is a queue at the ticket machine but may cost you if your flight is delayed beyond arriving that day.
So it will turn out whether you are willing to bet the money against the chance of a small or long delay and the chances of getting your train or an alternative one.