Administrative law exists because the United States Congress often grants broad authority to executive branch agencies to interpret the statutes in the United States Code (and in uncodified statutes) which the agencies are entrusted with enforcing. Congress may be too busy, congested, or gridlocked to micromanage the jurisdiction of those agencies by writing statutes that cover every possible detail, or Congress may determine that the technical specialists at the agency are best equipped to develop detailed applications of statutes to particular fact patterns as they arise.
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The regulations are treated by the courts as being as legally binding as statutory law, provided the regulations are a reasonable interpretation of the underlying statutes.
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It is important to understand that the CFR itself is written by lawyers for interpretation by lawyers and judges, and like statutes, must be carefully drafted in highly technical language to have effective broad application, yet limit the availability of loopholes. Unfortunately, the vast majority of employees of the federal government are not lawyers, and it would ask too much to force them to directly read, interpret, and apply the convoluted content of the CFR on a daily basis. Therefore, nearly all federal agencies have in-house counsel draft one or more internal manuals in plain English which set out daily internal operating procedures in very simple language that any layperson can follow. While such manuals do not really have the force of law, they are often the law as far as most employees and customers of such agencies are concerned, unless and until a dissatisfied customer of an agency appeals to a supervisor who does understand the CFR and the U.S.C. (or eventually sues the agency in court).
Oddly, despite the informality of such manuals, the U.S. Supreme Court has occasionally cited them as authority when confronted with situations not precisely addressed by the U.S.C. or the CFR.

No individual may enter a sterile area or board an aircraft without submitting to the screening and inspection of his or her person and accessible property in accordance with the procedures being applied to control access to that area or aircraft under this subchapter.