Mueller’s group illegally leaked this to the media.
They want to get all the information about Trump’s business which they will then leak to the media.
They will then go back for decades looking for something, anything which they can say is a crime.
Trump and his attorney’s need to fight this subpoena all the way to the Supreme Court if necessary.
Mueller has no authority whatsoever to subpoena Trump’s business records.
He likely does but even if Trump's business or financial dealings show some incentive or conflict of interest for a Trump presidency, Mueller obviously hasn't been able to produce on the collusion charge. So he's resorted to looking for anything to legit undermine Trump's presidency.