The problem is that when mixxx is running the audio output can't be switched to other audio from the computer and it also can't play the audio at the same time.
Depending on how Mixxx is used, this can be very cumbersome as you need to quit Mixxx to play other audio and restart it afterwards if you'd like to use it again.
For example, you can't take a pause of mixing music at home and watch a few videos in the browser or a movie in Kodi then go back to playing music (or just leave a set playing in the background). Another example is using Mixxx like a music player (an interactive one; especially if you use the auto-DJ feature for a playlist).
I'm running mixxx on Debian11/KDE with PulseAudio.