One oddity about Attorney General Merrick B. Garland’s decision to give special counsel status to David Weiss, the Trump-appointed U.S. attorney for Delaware who has been leading the investigation into President Biden’s son Hunter, is that both Mr. Garland and Mr. Weiss have already said the prosecutor was empowered to act independently.

That means making Mr. Weiss a special counsel may be more of a cosmetic gesture — essentially formalizing what has already been the case — than a new reality.

The attorney general’s move came against the backdrop of accusations by Republicans that Mr. Weiss had offered what they portrayed as a sweetheart plea bargain to the younger Mr. Biden because of political manipulations by Mr. Garland or by the White House. Functionally, the formalization of Mr. Weiss’s independence could serve as a shield against such accusations.

A special counsel is a prosecutor who wields the same powers as a U.S. attorney but is granted broader day-to-day independence from supervision. In making the announcement, Mr. Garland reminded the public that he had already said Mr. Weiss, who was appointed by President Trump, was operating outside the normal system of hierarchical oversight and control for the Hunter Biden case.

Discussing the decision to keep Mr. Weiss on to complete the Hunter Biden investigation in February 2021, after the Biden administration took office and most Trump political appointees resigned, Mr. Garland noted: “As I said before, Mr. Weiss would be permitted to continue his investigation, take any investigative steps he wanted, and make the decision whether to prosecute in any district.”

He added: “Mr. Weiss has told Congress that he has been granted ultimate authority over this matter, including the responsibility for deciding where, when, and whether to file charges and for making decisions necessary to preserve the integrity of any prosecution, consistent with federal law, the principles of federal prosecution, and departmental policies.”

In many ways, that description of Mr. Weiss’s previous authority does not sound notably different from the power he will now wield under the rules for special counsels, who also operate with day-to-day autonomy. In a statement on Friday, Chris Clark, a defense attorney for Hunter Biden, said the prosecutor’s new status did not appear to be a substantive change.

“This doesn’t change our understanding of Mr. Weiss’ authority over the five-year investigation into Mr. Biden,” he said. “For years, both Mr. Weiss and the department have assured us and the public that Mr. Weiss had more authority than a special counsel and full authority to negotiate a resolution of his investigation — which has been done.”

The differences between the rules governing U.S. attorneys and special counsels are less about law enforcement powers than they are about protections intended to bolster public confidence that they are operating free from political interference. For example, special counsels cannot be arbitrarily fired without a cause, like a finding of misconduct.

An attorney general may only block a special counsel from bringing a charge if the attorney general decides the step would be “so inappropriate or unwarranted under established departmental practices that it should not be pursued” — a high bar to meet. And at the end of the inquiry, Congress must be told about the decision, which creates a deterrent to abusing that power.

There is one notable requirement imposed upon special counsels: at the end of their investigations, they must write a report to the attorney general. While the rules envision that being a private document, the attorney general can choose to disclose it. Mr. Garland said he intended to release as much as possible of Mr. Weiss’s eventual report.

A factor in Mr. Garland’s decision not to name Mr. Weiss a special counsel from the start of the Biden administration, despite granting him broader independence, may be that the regulation envisions that special counsels will be people brought in from outside the Justice Department. Bestowing that status on a sitting U.S. attorney — Mr. Garland said Mr. Weiss would also continue in that role for Delaware — is unusual.

For example, Robert S. Mueller III, the special counsel appointed in 2017 to look into Russian interference in the 2016 election, was a retired F.B.I. director. Similarly, Jack Smith and Robert Hur, the special counsels investigating former President Donald Trump and President Biden, had previously been federal prosecutors, but neither was still working for the Justice Department when Mr. Garland named them to their current roles.

(It is not unprecedented, however. For example, Attorney General William P. Barr made John Durham, then the U.S. attorney for Connecticut, a special counsel to entrench him so he could continue to investigate the F.B.I.’s handling of the Russia inquiry even if Mr. Trump lost the election. Mr. Durham resigned as U.S. attorney in January 2021, but for several months he held both roles.)

Mr. Garland on Friday noted that Mr. Weiss had said in a letter to Congress last month that he had never requested special counsel status. But, Mr. Garland said, on Aug. 8, Mr. Weiss “advised me that in his judgment, his investigation has reached a stage at which he should continue his work as a special counsel, and he asked to be so appointed.”

Mr. Garland also said he had decided that taking that step now would be in the public interest. He did not say what had changed.

Republicans in Congress have stepped up their accusations recently that the investigation of Hunter Biden has been politicized, including taking testimony by an I.R.S. official, Gary Shapley, who claimed Mr. Weiss had sought special counsel status and had been turned down, an assertion Mr. Weiss denied. Granting him formal special counsel status could undercut such attacks.

But Mr. Weiss also unveiled a change to the investigation in court papers he filed shortly after Mr. Garland’s announcement:guilty plea negotiations had collapsed, he said, and that he now expected the tax charges against Hunter Biden — and possibly additional ones — to go to trial.