It's the buyer's call of course but generally things that have no obvious utilitarian value are bought for either of two reasons: future value (expected rise in value or expected keep of current value) or emotional reasons (such as belongings of close, personally meaningful people or beautiful items).
It's hard to imagine vibrant aftermarkets for these founding papers so it must be a combination of feeling closely connected to Apple's founders and huge amounts of money: enough for a million to become peanuts.
Still the money alone wouldn't explain it: I probably could pay one 1/1000th of my annual income (that'd be one million if I earned a billion each year) for those papers but I wouldn't because I can't think of a reason why.