If we analyse the sentiment a bit, we discover that there was no point asking for feedback - the course was already locked in and there was nothing to change. The feedback was useless.

Providing feedback has some risk for employees (maybe they'll get blowback) and it is easy for a manager to "ask for feedback" as a power play because they just want to show they can ignore it. It isn't nice to ask unless the intent is to treat the feedback respectfully and act on it.

I can imagine getting surprising feedback that can't be acted on. But that should be genuinely unexpected.

You might not know whether the feedback is worth acting on until you get it. If I ask my friend what he thinks about a new car I'm buying and he says "I don't like the color blue" then I might ignore it. If his feedback is that the transmission on this model explodes after 10,000 miles then I'll change my decision.

You might, but if you are your friend's manager instead of just his friend then you should take the opportunity to get better at asking for feedback to do a better job of managing the relationship.

Because what a manager should take away from that is that they weren't prepared enough - they could have asked "what do you think the most important aspects of buying a car are?" first and if "color" was the answer then either not asked for feedback or asked for feedback on something that would lead to a decision being made. The question was too vague.

If you ask someone for their opinion, learn they don't like blue cars, then ignore it and buy a blue car ... you could do more to be respectful there. You don't need to do special work to respect your friend - presumably the relationship is strong and they can just walk out if they don't think the respect balance is good. But really things are better if a manager respects their reports.