I already said what the benefit is. It's nice. Smoother graphics, smoother text, better graticule displays, being able to label everything accurately without taking up the whole screen. There are all sorts of things you can do by having a higher resolution. Just look at how they went for A1M instead of AC1M. That was likely a design choice limited by the display. They changed it because feedback told them users would rather have more descriptive label for the channel.
You don't need it, but there are things you can have with higher resolution displays you can't get with lower or with fewer trade offs compared to the lower resolution(DPI if you'd like, I'm assuming display size doesn't change). It's all a trade-off and I think Siglent did just fine but if I had a choice I would rather have a bit higher resolution than what they offer(1280x800 for a common resolution). As I said before I DON'T think it's a dealbreaker and it shouldn't be an issue for any of the SDS2000X Plus owners.