My naive understanding (correct me if I'm wrong) is that the
Constitution is already the supreme law and nothing can be above it,
nor Congress Resolutions. If anyone acts against the Constitution, it
must be already illegal. On the other hand, if someone is able to
break the higher law and get away with it, he could also safely ignore
In the face of repeated threats by the incumbent President to violate the law and not peacefully transfer power, the mere fact that the law says that someone is supposed to do something is besides the point. Judges simply have pieces of paper that the appropriate officials almost always listen to voluntarily.
But once the top official in the government signals he's going to disregard the law and by association the Courts, or implies that his three recent U.S. Supreme Court appointees will back him up if a dispute reaches the courts, out of loyalty to him and partisan sentiment (which even if not true, could sow fear and indecision in the hearts of people who might be afraid that he's right and that they will be pay a price for disagreeing with the President on this issue), reliance on government officials including the President whom government employees are usually required to obey, to obey court orders, isn't enough.
One of the ways to get a politician who could conceivably have the power to execute an extra-legal self-coup which is what the resolution is aimed at avoiding, is to get the overwhelming majority of the President's natural allies to commit to not supporting his extra-legal efforts before he starts engaging in them as he has threatened to do.
It was a risky move. The Democrats in the U.S. House basically forced the opposition to either disavow democracy which would disrespect the voters members of Congress will themselves be appealing to in a month and a half, or to take a vote that discredits and rebukes their own Presidential nominee in the same election. If Republicans in the U.S. House had backed the President as the five dissenters did, the vote would have set the stage for a likely self-coup attempt if Trump does not win the election.
This gambit was successful. Trump's extra-legal rhetoric was publicly rebuffed and rejected by all but about 5 members of his own caucus in the U.S. House. The measure was adopted in a bipartisan 397-5 vote. (A U.S. Senate vote on a substantially similar resolution, held September 24, 2020 was unanimous.)
Similarly, someone (probably senior civil servants who aren't politically appointed and senior military officers) has managed to get Trump's own political appointees in the Justice Department and the Department of Defense to publicly disavow his rhetoric about not making a peaceful transition of power, sending a message to rank and file soldiers in the military, and to rank and file career lawyers in the Justice Department, that on this critical issue, the President will not be supported by the people who in ordinary times report to him, if he tries to overstay his term of office.
The same individuals, had they not committed to a position pre-dispute and if they had not shared the information with each other authoritatively that no one supports this particular ploy of the President, might feel much more intense partisan pressure and fear to let the incumbent President disobey the law with impunity if first forced to take a position once the incumbent President's self-coup was already in the works.
A collective strategy of securing public pre-dispute commitments to not support the actions that the incumbent President has threatened to take gives everyone whose loyalties may be tested when a transition dispute arises confidence about which side will prevail in that dispute, because the worst outcome for any senior official or politician in a constitutional crisis is to appear to be a traitor to the political winner of the dispute. So its critical to influence the perception of who will win the dispute in the face of what amounts to gaslighting by the President over whether basic shared political norms like a peaceful transition of power are still in place. If it is manifestly clear to the incumbent President that he has no support from members of Congress from his own party, the military, or the rank and file in the Justice Department, for his plan, his incentive to never carry out the threat is much stronger.
Put another way, somewhat crudely, the purpose of the resolution is to insure that Republicans and other senior government officials don't perceive peer pressure to go along with a Presidential effort to refuse to transfer power peacefully.
More technically, it can be considered a "game theory" move, similar to a pact to address the "Prisoner's Dilemma" in which everyone participating comes out ahead if no one betrays anyone else in the pact, by having everyone commit to the pact before it is tested.