Just to be clear, it’s not like a protective order for their person (though Jack Smith and others already have to travel with large security details because of the stochastic terrorism of Trump and most other Republicans), but a protective order of the evidence in the case given through discovery. So before the trial the prosecution has to show the defense all the evidence it has, which is called discovery. The prosecutors here are concerned that Trump is going to leak that info in some way, like witnesses lists, so that his supporters can harass and intimidate witnesses on his behalf. Or maybe even bribe them or something. What the prosecution is seeking is a protective order to prevent trump from releasing publicly any evidence that they obtain through discovery. Normally there wouldn’t be anything preventing a defendent from releasing that info, though most sane people wouldn’t generally want their incriminating evidence released publicly. If the order is granted and Trump violates it, he could theoretically be held in contempt and go to prison where he no longer can violate the order.