One common reaction to the White House’s announcement on Wednesday that it was revoking former CIA director John Brennan’s security clearance was a natural one: What does he need a security clearance for anyway? He no longer works for the government. Why does he need access to government secrets?

When senior officials leave government — on good terms; former FBI director James B. Comey lost his clearance after being fired — there has been an informal standard of continuing to grant clearance for the rest of their careers. Those clearances go through a review process every five years, but it’s common. National security experts who spoke with The Washington Post outlined why that’s the case.

Consulting with sitting officials. Perhaps the most obvious is that former intelligence officials constitute a community of individuals with detailed, specific knowledge about past security incidents the country has faced. The United States faces recurring threats that often mirror past incidents. With regularity — every four or eight years, for example — much of the leadership of our intelligence agencies sees turnover. Former officials are part of the institutional knowledge of those organizations, and access to that knowledge can be useful.

“Having former senior officials hold active security clearances can be critically important for those currently charged with defending our nation,” said Jamil N. Jaffer, who was associate counsel to President George W. Bush and founder of George Mason University’s National Security Institute, “because it allows them to turn rapidly to people with significant experience, context and contacts to help interpret the activity of our opponents and to provide wise counsel and guidance, whether that’s in the terrorism, foreign policy or any national security context.”

Former officials don’t need a security clearance to offer their opinions to their successors, of course, but any counsel offered might be hampered if there’s current information that can’t be shared.

Serving the government in an official capacity. In addition to the informal outreach to former officials, there are governmental advisory boards on which those officials can serve in a formal way.

For example, there is the President’s Intelligence Advisory Board, which under President Barack Obama was co-chaired by Jami Miscik, a former senior CIA official who left service in 2005. Under President Trump, only one member has been appointed: billionaire investor Stephen Feinberg, who was made chairman in May.

Individual agencies have or have had similar boards. While he was director of the CIA, Leon Panetta’s external advisory board included former secretary of state Madeleine Albright, along with former military and elected officials.

Outside work might require a security clearance. In 2015, 4.2 million people held some sort of level of security clearance in the United States.

Why? Because many industries require security clearances. Clearances are common in the defense industry, for example; Lockheed Martin has 60,000 employees who hold a government security clearance. Intelligence agencies also frequently contract with outside companies to perform certain tasks, generally requiring that employees of those firms hold a clearance.

Not all former officials go into positions where they need clearance to carry out their job functions, but many provide consulting services for which a clearance is at least an asset.

Writing memoirs or histories. A not-uncommon post-retirement activity is writing memoirs about an official’s time with the government. In some cases, detailing certain events may require access to still-classified information. Such books are generally reviewed by the government to ensure that classified information isn’t included in the final product.

Several of those identified by the White House on Wednesday as under review over their security clearance have written books about their tenures — often books that are critical of Trump’s administration.

With the exception of Justice Department official Bruce Ohr, it’s not clear that any of the 10 people identified on the White House list actually rely on their clearances to make ends meet. Comey’s book detailing his time at the FBI, for example, was written after his clearance was revoked. Shortly after his own clearance was revoked, Brennan appeared on MSNBC to weigh in on the action, criticizing Trump and the administration in sharp terms. When the White House first floated the idea of revoking clearances, several of those who had been mentioned quickly noted that it would have little effect on their work — or that they didn’t hold clearances anyway.

Trump’s announcement inadvertently reflected on an unusual aspect of his presidency. The primary reason that presidents have supported ongoing clearances for departed officials is that it was helpful to their administrations. It’s useful for a president to be able to get input from those officials that can inform their decision-making. But Trump’s presidency has often eschewed that sort of advice, as reflected in part by the paucity of his Intelligence Advisory Board. A punitive, mostly symbolic action targeting his critics seems much more in line with his approach to his position than encouraging his subordinates to talk to experts and former leaders regardless of their political views.