It's just about inflicting as much damage on the Tories as possible ahead of an election through resignations (another today), defections, across the board condemnation of how Boris/Cummings are behaving and - crucially - Boris reneging on his unequivocal promise to take us out on 31 Oct.
The main damage being done to Labour right now is the charge that they are chickening out of an election. As soon as they vote for one, that charge becomes redundant. And then they can go into an election campaign as the party that will stop No Deal, rather than the party trying to stop Brexit.