I trust the AP News who wrote that it was the Biden election campaign. So this is one point for you. 
LOL, I purposely picked that one for that reason.
However, now the original question still stands, why would the Biden campaign think this was a good idea? It's not like Robert DeNiro is going to sway undecideds, and the location really adds more politics to a trial that is not supposed to be about politics, and only compounds the common view that this is lawfare.
If Biden really wanted to do a baller move, he'd pardon Trump instantly if convicted. That would actually help him more than having a washed up bitter old hollywood fool speak on his behalf. It would show no tolerance for politicizing the judicial system, which would actually get him votes.