He's not even on trial. I... don't understand the strategy here. "Trump did crimes, sure, maybe, but he's innocent! Hunter Biden though.... oh man..."
If, for some reason, the dam ends up breaking and they do vote to allow witnesses, the strategy will be to bring Hunter Biden in for the optics of questioning him on the Senate floor for the alleged "corruption" perpetrated by him and his father several years ago. The idea is that the Republicans can use this as an out to say "hey look see! The Bidens really WERE dirty, and doing exactly what the House is unfairly accusing the President of! They really were corrupt, therefore all the stuff that Trump did (which we're also saying he didn't do) was actually perfectly justified as rooting out corruption in Ukraine, and thus not impeachable (even though we're saying this conduct (which he didn't do) wouldn't be impeachable even if he did do it)."
Even if there is nothing factual to back up the Biden accusations, the mere image of having him there, answering to the Senate, is something they can plaster over Fox News for the rest of the trial's lifespan to justify Trump's crimes, use that as cover for his acquittal, and then continue using it in the hopes of hurting Biden's chances in the general election, just like the plan always was in the first place.