As someone who's been a Stock Android fanboy and got his first Samsung in over a decade, the only significant change for me is:
Keep pop-up windows open – Instead of minimizing pop-up windows when you go to the Recents screen, pop-ups will now remain open after you leave the Recents screen so you can continue what you were working on.
Password managers work perfectly well in popups. Calendars, stickies/notes, and other scensrios where the app isnt your focus but you need the utility of that app within another app.
In addition to what the other person wrote - popup windows are very useful on large screen devices like foldables and tablets. I'm on Fold 4 and the popup windows have been great for multitasking, like keeping an eye on a chat (or how far away your Uber Eats is) whilst you're doing something else in full-screen. Using a Fold is more akin to using a desktop, so multitasking features like this is much appreciated by folks like me.
Pulling down from top right corner to show the full quick actions menu. Now I need to swipe down twice to see all options, with this update only once. I will actually install the beta solely for this feature.
There's always the under-the-hood changes from Android upstream, and Samsung keeps adding customization options every update. Without rooting or 3rd party apps*, no other OEM comes close to OneUI's customizability. For me, that's a huge plus.
*I meant specifically 3rd party apps, because Samsung has the Good Lock suite of apps, which needs to be installed from the store. They work somewhat as a beta testing of features or advanced controls for existing features, and many of the features are added to new updates, for instance, the quick settings easy access up to One UI 5.1 (Android 13) is only available via Good Lock's Quick Star module, but seems to be built-in on One UI 6 (Android 14)