This impresses me as a pretty accessible discussion of what non-lawyers might perceive to be a complex legal issue.
Some people feel the “permanent” federal bureaucracy has too much power. That unelected career bureaucrats have too much say in the formation, interpretation, and application of laws. Approaches such folk advance range from narrowing/eliminating entire bureaucracies to making agencies subject to political (Presidential) control and influence.
IMO, such people often desire decreased regulation or corporate interests. Each person can decide whether that is a good or bad thing, or the extent to which other concerns ought to be weighed against corporate for-profit motives. Also IMO, Trump waged a pretty aggressive war against the federal bureaucracy, and each person can opine how well that went.
IMO, Congress lacks the resources and expertise to effectively do all that is needed through legislation. In case you do not realize (VERY GENERAL DESCRIPTION FOLLOWS), Congress enacts laws which are codified in the USCA - US Code Annotated. Take any Act of Congress you can think of - the Environmental Protection Act, the Patriot Act, the Social Security Act - it apears in the USCA. While most such Acts are quite lengthy, and can contain very specific provisions, in no way are they entirely comprehensive. They do not explain exactly how every provision is to be applied and enforced in every case, by whom, and by what process. They will generally contain a provision such as, “consistent with rules/procedures promulgated by the Secretary/Commissioner of xyz.”
Then the appropriate Agency promulgates rules for applying the provisions of the Act. These are contained in the CFR - Code of Federal Regulations. The CFR is quite lengthy, and gets down to the specifics. Generally speaking, Agencies make rules through notice and comment. They publish proposed rules in the Federal Register and invite comments from interested parties. Then, they publish their responses to the comments, and the final rules.
Based on your assessment of how well Congress works, you can make your decision as to whether Congress can/should perform this function.
Presidents appoint several persons at the top of most federal Agencies/Offices, but the vast majority of Agency employees are career civil servants. The political appointee has some considerable influence as to what Agency functions are stressed, but a HUGE part of what the Agency does simply goes on the way it always has been done, until the applicable regulations are changed.
Sorry if this is too long. Just wanted to give a quick assessment of my views and experiences as to how US federal Agencies work, and why some people feel it appropriate/desirable/necessary that they be afforded deference.