Talking to a friend today, she complained about someone wanting her help with a project when that person didn’t even know what the point of the project was.
Prima facie that does sound kind of objectionable. But is it?
People definitely do a lot of things without much explicit account of the point of each of them. For instance, many people go to university without taking a stance on whether it is primarily fulfills a learning function or a signaling function or an associating-with-elites function or or a getting the hell away from whatever they have been up to so far function.
It’s not that people think of going to university as pointless, they just have a vague sense of it being good in some rich array of ways.
Are activities motivated in this way much less useful than ones done with a distinct purpose?
I doubt it. But it seems complicated.
I’d guess that if a person thinks explicitly about their purposes in university, they might make university-related choices somewhat better. For instance, if they clarify with themselves that a big component is meeting like-minded peers, that might change which courses they do in counterintuitive ways.
But on the other hand, if they too much trust their breakdown of purposes, I expect things to get worse again: I’d guess a teenager who decides on a specific purpose for their university attendance to make worse choices than one just heuristically and hand-wavily trying to do what seems nebulously good.
Likewise, if I’m running a party, I don’t think it should have a point. Though I do think considering the possible sources of value from it might help.
On the other hand, if someone really wanted to cause a specific unusual event, and could give no account of why they wanted to do that except for evidently thinking it ‘would be good’, I might not be that excited to help them.
When should you know the point of your actions?

To the main point:
There is a transition point, a critical threshold between doing "generalised good-seeming stuff", concurrently and in parallel, and the point at which you can backward plan to specific good outcomes.
So the point at which you should know the point is when you can see how what you have been vaguely gesturing at with your imperfect intuition results in a specific outcome that you can backward plan to achieve from your current situation.
Generally, it's also strategically sound to continue to do the same generalised good-seeming things where they don't make your backward planning more difficult.
As regards to helping someone else who is confused, it seems to be that the help you would want to provide would be to help them be less confused.
You might be able to see that their intentions are in the right direction but there could be important things they are missing, not aware of, or it could be that the form of their understanding translated into a specific message would not land with its audience in a way that would accurately transfer the idea across.
Because so much of this is feedback-based, helping someone deconfuse by an IRL, face-to-face kind of conversation would generally be best, as then, presuming you are both truth-seekers, you would be able to quickly come to an understanding that perhaps neither of you could have reached on their own.
So my advice: listen to your friend and practice active listening, asking for clarification and encourage them to explain their thoughts more fully. Not everyone has years of writing experience backlogged to draw from to pick the perfect example in each circumstance to convey the right idea.
At the end of the day, the real question is: do you trust that your friend is genuine? If so, any project she thinks is good is probably worthwhile, at least a little bit, and if not, you learn that by this process of mutual clarification, and end up doing something else instead.
>On the other hand, if someone really wanted to cause a specific unusual event, and could give no account of why they wanted to do that except for evidently thinking it ‘would be good’, I might not be that excited to help them.
That's a fair call, and it's up to you to judge what exactly your friend might be getting at. There's not really any advice anyone can give you -- if you think the thing pointed at might be worthwhile, well you could help with the deconfusion, or else you could say, perhaps gently, no thanks, I think I see where you're going and wish you the best, but I don't want to be personally involved.
However, two heads are better than one and it's likely, again, that both together can come up with something better than either alone. I often find that I start with a basic plan to achieve something, then as I necessarily learn more in going about implementing it, the eventual sequence of actions both gets a much better result and looks nothing like that initial plan. It's kind of like making your plans compete with each other and picking the winner, periodically, a process which is known to "ratchet up" to out-of-normal-distribution results.,
Personally I hope you help your friend, because that's the default for friends, isn't it? The friendship relationship maybe has some degree of trust that, even if your friend is misguided, they'll listen and are fundamentally trying to work with you rather than against you.