Up until last year, if you plugged a pair of headphones into your Mac, the speakers were effectively disconnected. There was no way to send simultaneous audio stream to both headphones and speakers. Now, with modern T2-equipped Macs, you can double up on audio. For instance, you could have alerts sound through the built-in speakers, with music routed through the headphone jack, so you don’t get notifications interrupting your banging tunes.
The best part is that its really easy to set up. And, if you prefer the old behavior, you don’t have to do anything.