Hamilton thought it was superfluous. Federalist 74 says:
> “The President may require the opinion in writing of the principal officer in each of the executive departments upon any subject relating to the duties of their respective offices.” This I consider as a mere redundancy in the plan; as the right for which it provides would result of itself from the office.
Note that this provision must be redundant even without a unitary executive. Because otherwise, the implication is the only thing the President can do with principal officers is to ask them for an opinion.
Some modern scholars think the provision, though functionally redundant, is there to address a dispute that arose during the debates about executive councils: https://www.yalejreg.com/nc/reconciling-the-unitary-executiv... (“Unsurprisingly, the issue of an executive council arose at the Philadelphia Convention. Several proposals to create a council of state or a privy council were offered. Some of the proposed councils would have provided advice to the President but would not have required that he follow it, whereas others might have required that he secure the consent of the council. But each of the proposals was rejected. Instead, the Convention took language from part of one of the executive council proposals – ‘he may require the written opinions of any one or more members” of the council – as a model for the Opinions Clause.’”).
So the clause is there not to describe what the principal officers must do, but what the President need not do. The President may but does not need to consult his principal officers before taking action.
I don't buy this at all:
> otherwise, the implication is the only thing the President can do with principal officers is to ask them for an opinion.
That's not at all the implication... how do you even reach that conclusion? The obvious implication is that the president can only do what he is legally permitted, which means he could do whatever Congress provides for in law, in addition to what's in the constitution. Because, you know, his job is to execute the law. And Congress and the constitution are the ones establishing the legal framework for agencies.
> This I consider as a mere redundancy in the plan; as the right for which it provides would result of itself from the office.
It's nice that Hamilton thought that, but what did those who wrote it think? It seems safe to assume they wrote it for a reason, not as fluff. Which brings us to...
> Some modern scholars think the provision, though functionally redundant, is there to address a dispute that arose during the debates about executive councils [...] the clause is there not to describe what the principal officers must do, but what the President need not do. The President may but does not need to consult his principal officers before taking action.
It's great some modern scholars think this, but this also isn't compelling. If that's what they wanted... they could and should have just said that directly, not left it as a historical puzzle for people to speculate about.