Obviously there's a big divide between the opinion that no voter fraud happened in the 2020 Presidential Election, and that such widespread voter fraud happened in enough states so as to swing the election in multiple states. Realistically it seems obvious to me that a non-zero amount of people attempted to commit voter fraud, and a non-zero amount of clerical errors were made in the process of counting votes. Leading me to conclude that an unknown, but probably negligible amount of fraudulent votes would have been counted.
Using the most plausible sounding allegation I've read about for example: mail-in votes from deceased registered voters.
Some obvious possibilities are either individuals acting alone and mailing in the extra ballots they'd received in the post - then the counters not properly verifying the voter details, or simply a clerical error having marked of the wrong person as having voted.
If fraud is found, it allows for the system to be improved to prevent the same issue in future - given how long was spent investigating allegations of foreign interference in the 2016 election this seems like it's an issue for the Democrats as well.
If no fraud is found then it solidifies the legitimacy of the new Democrat Administration, and the public confidence in the system.
If a small amount of fraud is found, but would not have affected the outcome then the system can be improved so that it's not possible next election.
In the worst case (highly unlikely) scenario for the Democrats - widespread voter fraud occurred undetected by officials, but somehow is detectable after the fact - it would be impossible to determine with certainty which side had committed the fraud - so the only course of action would be to rerun the election.
If managed well to the effect of "we believe so strongly in the Democratic process, and in the legitimacy of this election vote that we support the investigation because we know we're the legitimate winners" - wouldn't the Democrats be in a situation where no matter the outcome of the investigation they'd be positively impacted? Or would it be simply perceived as too dangerous a political move to cooperate with the Republicans and risk it being interpreted by the public as a confession of guilt?