1080p video on a hi res screen

I don't know much about screens and videos so here goes a question. If the best video you can get is 1080p (ignoring 4k for now), does it look better or worse on a 2560x1440 screen, or any super hi res screen? Would it be better, worse or the same compared to a 1960x1080 screen?

If the video would be the same or worse, why would such a high resolution be good for people who like to watch video?

What about 720p video?

I ask because a lot of stuff I watch is 720p or 1080p, if I got a new tablet/phone with a higher res screen would it be worth it or would it be worse?