I think that Windows 2000 and Max OS were the pinnacle of clean Interfaces.
It seems like once the graphics capabilities became common, OS interface developers used them even when they shouldn't have. Remember when moving windows in Linux had a "jiggle" to them? The genie effect in OS X? What about the "cube" of multiple desktop? I still don't know what the point of transparent windows is.
A few effects are useful - like aero peek in Windows.
I think some of those animations make the desktop behave a little more like objects would in the real world.
For minimize, whether it's a line that shrinks, a genie lamp, a simple shrink effect, it's all the same. A simple visual cue telling you where it went.
A little form with the function is fine with me. There's a ton of computing power in even the weakest of machines. That said, I hope we'll always be able to turn it off.
Even a default install of Raspberry Pi OS uses a composited window manager on the RPi 4 now.
It can make things more intuitive. If you use mental models people already have, they tend to pick things up faster. I think the point of the cube, for instance, was to help people keep track of multiple windows a little easier by making it spacial as opposed to just a list. Maybe it wasn't very effective in that case, but I think the hypothesis was sound.
I think a lot of these things make interactions more pleasant, definitely more aesthetically pleasing. I know people love to hate on everything new but I grew up on early windows (my first PC was Windows 95) and I always thought it was kinda ugly but PCs couldn’t handle much more.
I think you can face-lift the W95/W2000 system without losing the usability, people just didn't do that. That's what I hate the most about "modern" UIs, designers make them look pretty and degrade usability, instead of just making them pretty.
It seems like once the graphics capabilities became common, OS interface developers used them even when they shouldn't have. Remember when moving windows in Linux had a "jiggle" to them? The genie effect in OS X? What about the "cube" of multiple desktop? I still don't know what the point of transparent windows is.
A few effects are useful - like aero peek in Windows.