The FBI Director is required to use government transport, including for personal travel, due to the need for secure and constant communication
. This rule, designated for high-level officials like the FBI Director and Attorney General, is a "required use" policy, though personal travel must be reimbursed to the government.
https://www.ainonline.com/aviation-news/business-aviation/2013-03-05/top-justice-officials-toed-rules-private-use-jets
https://www.gao.gov/assets/gao-13-235.pdf