Harris’ campaign...called for the mics to be left on for the entirety of the debate. The consensus, according to some reports, was that live mics could be a liability for Trump, given his history of interrupting debate opponents. Others argued that Harris, who spent a fair amount of time interrupting former Vice President Mike Pence during her vice presidential debate, was simply attempting to use the same strategy against Trump.

But President Trump does great, with mics / interruptions on.

Why would he -not- want them? We can only guess.

My guess is: To throw Kamabla off her game.

As quote above hints: xer needs to interrupt - whilst xer plays victim with xe "I'm speaking!" line or whatever xe line is - to seem strong at all.

Trump could handle xe easy, except, it might be even better to let xer hang in silence & discomfort.