How much MORE value am I adding?

The most effective way to kill anybody's commitment when they present an idea or work to us is to start suggesting changes. Now it's no longer their idea, and as a result their commitment will be cut in at least half.
A reduced commitment will in turn negatively affect execution, then outcomes, and ultimately impact. And we don't want to negatively affect impact.

So what we should instead think about is: how much MORE value am I adding? Unless it's an amazing margin (2x), the best course of action is just to say thanks, that's a great idea! How were you planning to execute it and is there anything that I can help with, now or later?

Alternatively, if we really want to add something we can ask: what kind of feedback or advice are you looking for from me? By asking in this way we're still leaving the agency - and the commitment - with them.

This does not apply only in employee - manager situations, but can happen in lower-stakes interactions too: think code reviews for example.