Nobody has been able to explain to me the relevance of Biden's testimony.
Well, playing Hannity, they think if they can create suspicion that Biden used his position as VP to help the son get his job and protect him from investigation, it will show that Trump was right in his concern for corruption to ask the Ukraine to investigate the Bidens. That mentioning the Bidens wasn't because he was a political rival but rather cause Trump was really focused on cleaning up the Ukriane as part of releasing the money
That is the supposed reason, in reality, it is just a deflection, and as I noted above, they see Biden saying something that they can turn into umpteen conspiracies to discredit him politically