But also, consider the marginal utility of a unit of goods or stuff that can be traded for goods. A rich person (caveman or investment banker) can only get so much out of another hunk of mammoth or round of gold, respectively. But if he gives it to his fellow man, that would have starved to death otherwise, his fellow man can stand beside him when the sabre tooth tigers attack.
Possibly, but how do we know how much utility the rich caveman is getting from his extra hunk of mammoth, how much he'll get by giving it to the poor caveman, or how much the poor caveman will get (perhaps, only the utility a meal)? Perhaps the poor caveman will simply eat the hunk mammoth, ask for more, and become more and more dependent on the rich caveman's generosity, or perhaps the poor caveman will use the energy obtained by eating to attempt to collect his own food or otherwise better his position. How do we know the poor caveman will stand beside the rich caveman gets attacked by the sabre tooth tigers, rather than run out of fear, or even try to hinder the rich caveman's escape in the hopes of him being killed and taking his remaining stockpile? (Just as an aside, if the poor caveman is incapable of getting his own food, how much help would he be in fighting off sabre tooth tigers?
) And who should decide whether or not the rich caveman gives his extra hunk of mammoth to the poor caveman?
My point is it makes a difference whether the transaction is voluntary or coerced. If it is voluntary, the rich caveman is presumably getting more utility by doing so than by keeping the extra hunk of mammoth. If it is coerced, that may not be the case, and then the rich caveman no longer has any incentive to obtain that extra hunk of mammoth. There is no longer any reason for him spend the resources (time, energy, weapons, etc.) and take the risks (injury or death) to obtain that extra hunk of mammoth that will now just be taken from him. He's much better off simply using those resources for something else that benefits him.
I recognize there is some overlap, and some coerced donations may have been made anyway in the absence of coercion.