If you develop apps on your Mac, or work on websites, then being able to view “invisible” files is useful – although viewing them all the time can clutter-up Finder and make it hard to find what you’re looking for. Therefore, being able to switch on and off viewing them is a God-send.
It seems that macOS Sierra brings a keyboard shortcut to Finder (and file open/save dialog boxes) that lets you switch on and off viewing hidden files instantly, with no prior setup required. It’s this:
If you live outside the US, you might describe the shortcut as Shift+Cmd+full stop. This keyboard shortcut has always worked in file open/save dialog boxes to show invisible files, but the fact it works within Finder windows is entirely new to macOS Sierra.