No major benefit of buying in advance it's not like the UK system, what you pay in adavance us pretty well what you pay on the day,
Depending which train you get, cost is circa between 13-15 euros each way & there's no big saving for a return.
If you want to purchase in advance then buy direct from the Dutch site ns.nl ( don't use trainline.com international as you'll pay about double, and if you get an e-ticket you say a Euro per ticket. Pay an extra 2.60 euro means you can go on an express train and journey time around 25 mins, instead of 50 mins. Screen shot below with example info: