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.

I like pop-up windows. Especially on my fold but I also enjoyed them on the Ultra just not as much