I've often wondered this.
I'd be happy with what my dad used to call "eff you" money (although he didn't say 'eff'), in other words you have enough money in the bank to last the rest of your life, and possibly the life of your kids, and are not beholden to anybody.
If you have a billion in the bank (a thousand million pounds), I'd say it's pretty un-spendable, although I'd like to be in a position to give it a bloody good go.
People can amass as much wealth as they can, and use it as they choose, and bloody good luck to them in my view, and I'd never be one of those people who try to take the moral high ground and moan that they should be using the money for good causes; their money, their choice.