I notice that people seem to scan their tickets twice, on entry and exit, on the NS system (Intercity / Sprinter) in Amsterdam. Since the officials scan your ticket on the train, what's the purpose of scanning on the platform? I've tried with my one-way tickets and usually the scanner beeps and flashes red, though once it flashed green.
You can travel the NS train either by buying a single ticket, or by using an OV-chipcard (Dutch: "OV chipkaart").
If you buy a single ticket it doesn't matter.
If you travel with the OV-chipcard (as most people do), you pay for the distance you travel. The system needs to know how far you've traveled so you "check out" when you leave the platform. The official who checks your ticket or card on the train just does so to check if you have a valid ticket (but obviously doesn't know at which stop you'll get off).
Note that a single ticket is always €1 more expensive than traveling by OV-chipcard. An OV-chipcard is usually €7.50, but sometimes they're discounted. So depending on the amount of traveling you're doing, buying a chipcard might be more economical (they're valid for 5 years).
You can only exit the station when you scan or swipe your ticket. Here in Rotterdam but more and more stations in the Netherlands are getting this system.
Those people are scanning their 'OV chipkaart', and not their one way train tickets. Those cards are either prepaid or subscriptions and they need to check out to be billed for the actual trip they made. Failing to check out means getting charged more (because you might have made a way longer trip).
With a one-way ticket there is no need to check out.
Most people don't have tickets anymore but a pay-as-you-go RFID card like in many public transit systems (à la Oyster card). So you validate once to check in and another time to check out and determine the price of the ride.
Of course, for single use tickets (paid in advance) it does not matter so much but some stations (e.g. Rotterdam) now have automatic gates like some subway networks, which means you will need to keep your ticket and "check out" as well. Amsterdam is still open for the time being but you must have walked through those gates as they are already in place.
The "OV" as they call it can be used to determine your location of departure and location of arrival, so the price for your trip can be calculated automatically and can be subtracted from the "saldo" on the ov-ticket.
So for example your location of departure (check-in) is Amsterdam Central. Let's say your going to Haarlem. At arrival in Haarlem you scan again (check-out), now your destination is known and the right amount of money can be subtracted for your trip.
The reason that the officials scan your ticket on the train is, because they want to check whether you scanned your ticket at departure and so make sure you are not enjoying a free ride.
Have fun in Amsterdam!