Why do DJs wear headphones on one ear?

While I take a break from my live streaming, I've been soliciting "Why?" questions about DJing. A simple question about the quirky way DJs wear their headphones turns into more questions about the practice of beat matching, and when it may or may not be appropriate.
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[@StillWatersRunDeep101]

I put you on .75 speed and the speed of your voice was great, no offense, you speak quickly

[@djphiliptan]

Good feedback, you're right!

[@naturebackgroundsounds]

Can you mix and cue with the headphones on both ears…. I mean that’s why the technology exists. I remember the 1980s when Grandmaster Flash perfected what he called the “peekaboo “ method because the cueing technology didn’t exist so he actually needed to listen to both tracks simultaneously. But you are extremely knowledgeable in your field. Thank you for sharing the “facts”

[@djphiliptan]

Yes, you can absolutely mix and cue with headphones on both ears, if you prefer it that way. GMF's peekaboo actually only allows him to hear one deck at a time on his headphones, but it was perfect for his style of quick-mixing. Current manufacturers of "battle"-style mixers have pretty much standardized on GMF's side-to-side switch. Club mixers tend to stick to the older disco paradigm of push-button cueing for each stereo input.

[@djphiliptan]

Oh, and here's a great photo of the peek-a-boo wiring! twitter.com/DJFlash4eva/status/600332233660960769/photo/1