Not only that - he's got many other criminal counts still pending. The potential witnesses to those crimes are paying close attention to how much protection these witnesses get in the wake of their testimony.
Lift this gag order, Trump goes on a rampage, witnesses get an increased level of threats or, god forbid, actual violence - that is a chilling effect on witnesses in all of Trump's other cases, and to a lesser extent, for witnesses in any future cases involving any defendant.