This is all about delegitimizing the impeachment process itself. Trump getting impeached twice was historically relevant because it stood as tangible evidence of him being one of the worst presidents in history and easily the worst of the last 50 years. The Republican narrative remains that those impeachments were politically motivated and not based on any valid legal or ethical concerns. So now they're going to do what they accused Democrats of doing and deliberately make a mockery of impeachment.

The point of this isn't to actually punish anyone in the Biden administration. It's an inconvenience at worst and they know it. The point is to make impeachment a joke. Something one party in Congress does to the other in the White House as a regular and inconsequential thing, just part of the theater of it all. This retroactively defangs Trump's impeachments in the eyes of people on both sides who aren't dialed into politics and really only pay attention every 4 years and skim the occasional headline. It also preemptively reduces the gravity of any future impeachments of Republican Presidents because impeachment becomes routine and therefore mundane to the average person.

Basically the Republicans don't like checks and balances that require them to behave like civilized, functional adults doing their jobs, so they're just going to smear their shit on those checks and balances until the voters no longer take them seriously and forget they ever had any real meaning. And it will work too.