In OS X a tool/terminal command was required to show/hide the hidden and system files.
Various tools like TinkerTool, Cocktail and various others could do that.
A restart of the Finder was always required.
Now with macOS Sierra these tools are no longer required to show/hide hidden and system files.
You can now easily toggle between show and hide the hidden and system files with the key combination Command + Shift + .
No restart of the Finder required anymore.
I thought this has been possible for quite a while?
http://lifehacker.com/5362911/cmd%2Bshift%2Bperiod-toggles-hidden-file-visibility-in-snow-leopard
ah, maybe not 
or at least it was only possible in open/save dialogs, but not in Finder (in pre-Sierra):
Yes and no.
It was possible within Open/Save dialogs but not in the Finder.
Since Sierra you can use it in the Finder too.
A few days ago I was working in El Capitan and wanted to see the hidden files and used the key combination.
It just didn’t work. 
Eh, how do I use this keycombo? Does nothing…
Are you using macOS Sierra?
For example, go to your home folder.
Press the Command and the Shift key and then press the . (point/dot)
You should now see various hidden files and folders.
Press the same key combination and the files and folders are hidden again.
Running High Sierra. Apple has cleaned up the old .DSStore (name?) files of the olden but golden times. That confused me. All well now.
yup, works in the finder here on regular Sierra, not HIGH sierra. What fun.
“Oops, we weren’t supposed to do anything useful.”