These are always really neat to hear. Artist sometimes offer up vocal only versions of their songs. When you strip away the music, it lets you hear how intricate the vocals can be, layered on each other. Gives another perspective on a song you already know.
Dan + Shay gave us one of those here with Tequila:
I've had some other favorites over the years. This was one of the first times I saw this done, and it's impressive. Van Halen's "Runnin' With The Devil." Love Diamond Dave's performance here, and that you can hear the music playing from his headphones.
I've got to include one from one of the best singers of all time. Freddie Mercury was amazing. Nobody can sing like this, nor probably ever will. Queen's "Don't Stop Me Now."