Trump claims his call to Raffensperger was "perfect," and prosecutors think so too: they're calling it "incredibly strong evidence" to support criminal charges. Trump was the leader of the free world when he made that call, and he demanded that Raffensperger 'flip' the election or face potential legal consequences, which is obviously a crime.
If Willis thinks Trump's phone call is going to lead to criminal charges, she's naïve. The call proves that Trump was making sure Raffensperger was doing all he could to make sure the election was fair, even if the former president didn't do it tactfully. This grand jury is a waste of taxpayer money and court resources.
The DOJ and state DAs like Fani Willis are all playing catch-up. They have likely heard rumors about the damning evidence that the Jan. 6 Committee will present at public hearings starting next week and - in anticipation of a public outcry - they are finally taking action to hold the former president and his allies accountable.