Reporting since the raid has centered around allegations that Trump had taken classified material from the White House to his private residence after his term in office ended—a violation of federal law. The National Archives and Records Administration said as much in a letter to the Department of Justice (DOJ) earlier this year.
Many Republicans have fulminated against the raid, calling it "next-level Nixonian," akin to the acts of "3rd world Marxist dictatorships." Minority Leader Kevin McCarthy of California warned that the DOJ "has reached an intolerable state of weaponized politicization" and that when Republicans retake the House of Representatives after this year's midterm elections, they "will conduct immediate oversight" of the department.
In the press conference, in which he spoke less than five minutes and took no questions, Garland announced that in accordance with his pledge that the department would "speak through its court filings and its work," the DOJ had filed a motion to unseal the "search warrant" and "property receipt" from the Mar-a-Lago raid. Garland indicated that the DOJ had moved to make the documents public "in light of the former president's public confirmation of the search, the surrounding circumstances, and the substantial public interest in this matter." He did not address what materials were seized, but the motion does cite statements from Christina Bobb, Trump's lawyer, that the warrant was for "presidential records or any possibly classified material." "As such," the motion states, "the occurrence of the search and indications of the subject matter involved are already public."
While he clarified that the search warrant was "authorized by a federal court upon the required finding of probable cause," Garland admitted that he "personally approved the decision to seek a search warrant in this matter." He also clarified that the DOJ "does not take such a decision lightly. Where possible, it is standard practice to seek less intrusive means as an alternative to a search and to narrowly scope any search that is undertaken."
The New York Times reported earlier today that Trump actually received a subpoena earlier this year, requesting that he turn over classified material that had been improperly taken out of the White House. While Trump apparently returned material in response to the subpoena, the Times alleges that federal investigators felt there was more. The search warrant was seen as the last straw after other less invasive methods were attempted.
It's entirely possible that the FBI, not known for its dedication to transparency, is only making this move because of the massive spotlight that Trump and his supporters have put on it. But regardless of the motivation, more transparency in law enforcement is a net positive.