I guess the reasoning for SAS Plus over DY top fares could be lounge access (where applicable), the benefits of frequent flyer programs, free snacks and drinks, luggage allowance (particularly if you have *G or SK status) if you need it, and priority counters for checking that luggage in (still only if you need it)

The market dynamics have pulled SK closer to an LCC, and meant that Norwegian is bit less hardcore LCC than some, blurring the lines.