One big reason is that he can time the actual donations, for tax purposes. That allows him to make big donations in years where he has large amounts of taxable gains to offset. If he donated $1B this year, he'd likely hit a deductibility cap (especially if he donated to a private foundation, as opposed to a generic 501(c)(3)).
https://www.nytimes.com/2015/12/03/technology/zuckerbergs-ph...
Also, of he's giving the money to his LLC, is this deductible at all?
This is a dynamic system, so changing one parameter would cause other parameters to be changed as well.
Of course, a savvy parent would put pre-IPO stock into a trust, using up some portion of the $5-10 million exemption amount (it's up in the air because some of the Trump tax cuts are not permanent). Millions of dollars in pre-IPO valuation stock turns into tens or hundreds of millions by the time you die. And there are more nuanced (and/or aggressive) strategies that can yield greater results — or at least that's what I remember from my trust and estate law class.