My guess is that they have a sequence of events they want to prove took place, and they also want to prove that Trump knew what he was doing was wrong at each step. So maybe the first thing is that there was a plan in advance to subvert the election, and they get testimony from a few different people who were at a meeting about it, another couple people who were on an email discussion, and someone who Trump spoke to personally. Do that kind of thing for every step in a long chain, and it can add up. But any weakness in the chain could be grounds for reasonable doubt, so they have to make sure it’s all solid.