The standard macOS interface has quite a few semi-transparent elements, which like frosted glass provide a glimpse of what’s beneath them. At Apple events, execs go giddy about how pretty this is. In use, these elements vary from being distracting to outright dangerous. For example, if you have a motion-sickness issue and an animating web page is sitting behind a semi-transparent element, it can take a while before you realise it’s affecting you, by which time it’s too late and you’re already dizzy.
“Fine”, says Apple, grumpily, “so just turn on Reduce transparency”. Only it’s not that simple. Because when you do, Apple designers get in a strop and hurl logic out of the window. What you’d expect to happen is for macOS to remove the semi-transparent bits. So instead of Finder sidebars or the macOS app switcher showing what’s beneath them, they’d just have a neutral solid background. Nope. Instead, in its infinite wisdom, Apple’s decided those components should instead be coloured by your Desktop background.
You can see what he means by clicking through this gallery I made showing just how much the desktop background sets the tone for UI elements, even with reduced transparency enabled.
If you use a photograph as your background, macOS samples the colors present to tint its UI elements. It can lead to some pretty awful-looking windows:
The old ways, sometimes, are just better.