That’s not a stupid question, it is a good question and a valid one. First, I am not a lawyer. However, my layman understanding is this:
In a jury trial there are strict rules regarding testimony specifically so that information is not introduced into direct evidence or via testimonial evidence that could serve to unduly prejudice a jury. Obviously ALL testimony is prejudicial in some capacity in the sense that it is intended by its very nature to change the opinion of the fact finders in a legal procession, in this case a jury and a civil trial.
These rules for what can or cannot be introduced into evidence via testimony are litigated before trial, and even during trial because there are often disagreements between the parties about what constitutes unduly prejudicial testimony. The judge rules on that, which determines what can and cannot be introduced.
If Donald Trump was allowed to bloviate about anything he damn well pleased on the stand, especially regarding information that had already been ruled inadmissible, his own lawyers could then turn around and call for a mistrial on the grounds that the judge had already ruled that information to be inadmissible. Granted that is unlikely to happen, but it would not be unprecedented nor does the judge want to leave that as an ambiguous issue on appeal.
So, in order to maintain fairness in the legal process the judge preordaines what information a jury can hear in order to avoid that situation in the first place.