For me it's the efficiency. The 90's era Windows UI and applications ran productively on machines with 8MB of RAM. That's an enormous amount of value for what amounts half today's desktop CPU cache. The limited memory forced developers to conform to the OS provided API and resources (fonts, color pallets, image formats, etc.) and the result was small, fast, consistent applications.

Today this discipline survives in mobile devices. Meanwhile, our desktop operating systems are festooned with a multitude of widget toolkits and language runtimes yielding wildly diverse, inconsistent and often fragile behavior and require 3+ orders of magnitude more memory.

Aside from (usually) pretty nice font rendering, what has actually been achieved with all of this?

I don't really think it is only that. My first OS was DOS and i used Win3.1 a lot and while i do have nostalgia for those two, i think Win9x was better in terms of GUI design and as others have mentioned, the peak was around Win2K (though not everything was going upwards, i still think border-on-hover introduced in Win98 was backwards and essentially the herald of featureless form-over-function modern flat design).

And ever since i started using a window manager in Linux (somewhere in early 2000s) i really liked Window Maker's style even though i never used NeXTSTEP for many years (in fact the only time i used it i was kinda disappointed at how primitive it was in terms of window management compared to Window Maker :-P).

Absolutely