Charitable foundation are tax free havens
From Soros' mouth, one of the reasons foundations are set up for:
to avoid paying estate or gift tax on your inheritance
From "Soros, The Life and Times of a Messianic Billionaire", pp.170-171:
By the time he had established the Open Society Fund, though his plans for it were modest, vague, and unrealized.
The fund was set up in 1979 as a charitable lead trust. With customary candor, Soros describes the motives for creating the foundation as basically selfish: "A charitable lead trust is a very interesting tax gimmick. The idea is that you commit your assets to a trust and you put a certain amount of money into charity every year. And then after you have given the money for however many years, the principal that remains can be left [to one's heirs] without estate or gift tax. So this was the way I set up the trust for my children."
At the outset, his idea was that he would contribute up to $3 million a year for twenty years.