I don't believe anyone has successfully answered 'what are the foundations for'. So I'm going to go out on a limb with some lateral thinking:
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the tax arrangements around Foundations date from the era of the two Roosevelts. They are a Progressive Era 'socialist' mechanism to force inherited wealth from the Gilded Age into 'philanthropy'.
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Clearly, they are used to preserve wealth in families, in a particular way. But that is only because the law is framed that way – what is the purpose of that law? What is its real purpose?
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Might they also not be used to control that layer of elite family, by holding their wealth, their family fortune, and thus the fortune of their families hostage?
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All the foundations tend to the same end, no matter what the families want – this suggests that either 1/ networking and control eventually causes them all to share a common view point or 2/ control and forcing and agenda is the point of them
Foundations exist not to serve the likes of Gateses or Clintons – but to put them on a leash. Foundations are the strings, and the means by which elite families of a certain level are controlled. They control the family members by making their wealth and access to power conditional on serving whatever agenda is specified. It is easier to control a board of directors than to control blood-related family members, who will conspire against you.
My tl;dr answer:
Foundations are the links from the visible 0.1% upwards to their controllers.