The main thing is that Kerri needed it herself. Her ambition was to make the Olympic AA final and after her first vault, she was 4th on the US team.
As for the team, they didn’t require Kerri’s hit vault in the end, but nobody would’ve known that at the time. Russia still had one score to come up and two more to compete after that. I remember once working out that they’d have needed something like a 9.818 average from the final three routines to pass the US if Moceanu’s 9.2 had counted. Floor had been a high scoring event that night, with Romania and the US both racking up the 9.8s. If anyone had been paying sufficient attention, and tbf this would be a really difficult thing to keep track of and work out in real time, it would’ve looked like counting a low score on vault could lose the US the team title.