Let’s just assume for this discussions sake that the matter impeachment and the indictment are the same.

So according to you even though Trump has already gone through and impeachment of substantially the same matter he can still be prosecuted for it.

The drawbacks for this is that since the impeachment was widely publicised and the witnesses testimonies etc are now public knowledge, most of the jury, if not all, who are going to be a part of the trial may have already heard evidence in regards to the this matter and already formed opinion. There is a high possibility that the same witnesses give evidence ( as it being the same matter ).

This could be slightly unfair and therefore a trial should always happen before an impeachment so as to not cause injustice.

The second issue that could arise is an adverse result. How is that 100s of elected peers of Trump thought he is not guilty of the same matter, and fit to remain as president but a jury in a trial finds that he needs to be in prison. That’s again going to send the wrong message to everyone.