Sharp, clear and legible text even at small font sizes. Once you’ve gone “retina” you won’t go back.

The smartphone market has already declared high DPI the winner here; desktops are lagging behind only due to inertia.

The main thing you gain is crisp and legible small point text for displaying large amounts of code, terminal outputs, etc without taking up large amounts of desk space. I have both standard DPI (~110) and higher dpi (180-220) displays all sitting next to each other at my desk right now. The high res ones look drastically better and are much easier on the eyes. It is quite noticeable.

This resolution is not particularly crazy either, it is the standard DPI of all of Apple's gear for the last 6 years or so and much lower resolution than most tablets & phones. One of the bottlenecks for moving this tech beyond Apple laptops and all-in-ones (of which I am not a huge fan) has been the lack of standard external connections with enough bandwidth for these displays.

On top of people mentioning text, I'll add chat emotes to the discussion. If you use anything like Twitch chat, it's filled with custom emotes you've never seen before. I can read text without each character having an abundance of pixels. Similarly sized icons turn to blobs at that size. High DPI makes those icons very clear even when tiny. I notice a huge difference between my 1080p monitor, my phone, and retina mbp.

The sheer improvement in clarity of text is massive - makes it well worth it on it’s own, imo

Text is sharper, and diagrams with fine lines. PDFs using the standard Computer Modern font look much better onscreen on the retina imac in my office than the mid-2011 imac at home.

Everyone is talking about text but there's icon crispness as well. A non-power of two rescaled of pixel perfect icons can look terrible.