There are scenarios where Congress might have to decide who should become President, so it has to be in place before Inauguration Day. For instance, if Biden wins Georgia but Trump wins the other States, both will have 269 Electoral College votes and the House of Representatives will decide the election. Unfortunately, Trump would almost certainly win in that scenario.
Another scenario is that Pennsylvania goes to Biden and is the only State that takes him to 270 Electoral College votes. If the Pennsylvania legislature claims electoral fraud, it could send its own pro-Trump slate of electors to the Electoral College. In all likelihood, that would send it to the new Congress to decide.
Of course, there’s now no good reason for delaying the sitting of the new Congress until the New Year.
Another scenario is that Pennsylvania goes to Biden and is the only State that takes him to 270 Electoral College votes. If the Pennsylvania legislature claims electoral fraud, it could send its own pro-Trump slate of electors to the Electoral College. In all likelihood, that would send it to the new Congress to decide.
Of course, there’s now no good reason for delaying the sitting of the new Congress until the New Year.