It's not about real estate, but rather about sharpness. At 3820x2160 ("4K") you can double every pixel, when compared to 1080p. That means that text at the exact same "size" is twice as sharp, producing less visual strain and improving readability (especially for fonts with complex strokes)
The difference is absolutely striking: I will never go back to a non "retina" display if given the choice.
It’s also real estate. I have the LG 5K 27” monitor—which I like, by the way, I don’t know why people give it crap—and routinely wish it was just a touch larger.
My screen is filled with Excel and some sort of online database, usually. I’m also using my 13” MBP screen below this one for chat windows, email etc.
When doing photography work, you can look at image thumbnails and actually judge sharpness and colour directly off them. And when filling the screen, you can have a much better idea of how they will look in print.
As for text, I've been able to use tiny font sizes and increase the information density since hi-DPI screens— my eyes are good for it. Ahh, iPhone 4 and retina MBP... they were astounding tech at that time.
I cant deal with 1080p anymore, and large (27"+) 1080p monitors make no sense to me. Maybe they're good for people who need to have physically large text etc. but I find it useless for fitting information densely on the screen. I for one would rather use my 13" MacBook pro laptop screen than a large 1080p display. (I use a 24 inch 4k monitor and it's pretty great.)
The difference is absolutely striking: I will never go back to a non "retina" display if given the choice.