It's always nice to be pleasantly surprised by an unannounced tweak to a feature, especially if that feature is on your list of "software annoyances that must be silently endured." By their nature, these small changes don't always make themselves immediately obvious. For example, I'd been using El Capitan for months before I noticed that anything was different about the Finder sidebar.
The OS X Finder is a fertile ground for complaints — I won't catalogue mine — but its insistence that windows should have sidebars was near the top of my list. They occupied screen real estate and offered little functionality that interested me. Furthermore, when a window suddenly gained a sidebar, its dimensions wouldn't change, pushing columns out of view or even truncating filenames. Hiding the sidebar involved a lavish animation, but even worse, a predictably unpredictable change in window size: the outside bounds of the window would shrink, but the content area wouldn't stay the same.
I tried to work around this in several ways, including some abortive attempts at Keyboard Maestro scripts. A combination of the Hide sidebar menu command with zooming the window went nowhere, mostly because the Zoom command was equally confused about what proper dimensions for a Finder window should be. (After years of failing at this where the Classic Mac OS had succeeded before, Yosemite finally gave up on the concept of zooming, assigning the green window widget as a fullscreen toggle instead.) In the end, all I managed to do was to simplify the keyboard shortcut from ⌥⌘T to ⌃T.
So, imagine my delight when I hit my custom ⌃T shortcut in El Capitan and — whoosh! — the sidebar flew out of the way nice and quickly. And wait, what's this? The window dimensions remain the same! A genuine grin broke across my face when I realized what had happened. I kept toggling back and forth, like a child discovering the joy of a new toy. Yes, I probably have years ahead of Finder sidebars showing up where I don't expect, but when they do, they'll be hidden easily.
Making this change was not a priority fix, by Apple's standards — it's about as far from a crasher as you can get. But somebody on the Finder team must have run into the same obstacles I did, and wanted to make using their software a little more joyful. In a demanding product release cycle, it found its way in, and it's made a difference.