This is significant because the New York Attorney General has the power to file state criminal charges against anyone in Trump's orbit who is found to have violated New York state criminal laws. The New York Attorney General already had an ongoing criminal investigation into Trump and associates on other issues.
While Trump can pardon anyone charged with or convicted of federal crimes, he has utterly no power to pardon state charges.
This essentially means Trump has just lost his trump card. He can't potentially silence people with pardons. He can't hobble a federal criminal investigation when pardoned individuals could also face state criminal charges.
Trump also cannot pardon himself if he's facing state criminal charges.
This scandalous drama just got a lot more interesting.