With Snyder, the bottom line always seems to a factor. So beyond the perceived PR benefits, I'm guessing that it's not a coincidence that he set up this foundation at a time when the stock market is performing so strongly.
Per Wikipedia:
"The Tax Reform Act of 1969 defined the fundamental social contract offered to private foundations. In exchange for exemption from paying most taxes and for limited tax benefits being offered to donors, a private foundation must pay out at least 5% of the value of its endowment each year...Administrative and operating expenses count towards the 5% requirement; they range from trivial at small unstaffed foundations, to more than half a percent of the endowment value at larger staffed ones. Congressional proposals to exclude those costs from the payout requirement typically receive much attention during boom periods when foundation endowments are earning investment returns much greater than 5% (such as the late 1990s); the idea typically fades when foundation endowments are shrinking in a down market (such as 2001-2003)."